feat: allow html tags to be replaced with icons

## Details

Request: #336

By default no tags are configured so nothing happens.

If matching tags are found the start & end tags get concealed and the
configured icon / highlight are inlined at the start.

Example configuration:

```lua
require('render-markdown').setup({
    html = {
        tag = {
            file = { icon = '󰨸 ', highlight = 'Normal' },
        },
    },
})
```

Author: MeanderingProgrammer

README.md

@@ -682,6 +682,11 @@ require('render-markdown').setup({
             -- Highlight for the inlined text
             highlight = 'RenderMarkdownHtmlComment',
         },
+        -- HTML tags whose start and end will be hidden and icon shown
+        --   The key is matched against the tag name
+        --   'icon':      Gets inlined at the start
+        --   'highlight': Highlight for the icon
+        tag = {},
     },
     -- Window options to use that change between rendered and raw view
     win_options = {

doc/render-markdown.txt

@@ -1,4 +1,4 @@
-*render-markdown.txt*         For 0.10.0         Last change: 2025 February 17
+*render-markdown.txt*         For 0.10.0         Last change: 2025 February 19
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*
@@ -743,6 +743,11 @@ Default Configuration ~
                 -- Highlight for the inlined text
                 highlight = 'RenderMarkdownHtmlComment',
             },
+            -- HTML tags whose start and end will be hidden and icon shown
+            --   The key is matched against the tag name
+            --   'icon':      Gets inlined at the start
+            --   'highlight': Highlight for the icon
+            tag = {},
         },
         -- Window options to use that change between rendered and raw view
         win_options = {

lua/render-markdown/handler/html.lua

@@ -19,9 +19,16 @@ function Handler.new(buf)
     self.config = state.get(buf)
     self.context = Context.get(buf)
     self.marks = List.new_marks(buf, true)
-    self.query = treesitter.parse('html', '(comment) @comment')
+    self.query = treesitter.parse(
+        'html',
+        [[
+            (comment) @comment
+            (element) @tag
+        ]]
+    )
     self.renderers = {
         comment = require('render-markdown.render.html_comment'),
+        tag = require('render-markdown.render.html_tag'),
     }
     return self
 end

lua/render-markdown/health.lua

@@ -5,7 +5,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '8.0.4'
+M.version = '8.0.5'
 
 function M.check()
     M.start('version')

lua/render-markdown/init.lua

@@ -41,8 +41,13 @@ local M = {}
 ---@field public text? string
 ---@field public highlight? string
 
+---@class (exact) render.md.HtmlTag
+---@field public icon string
+---@field public highlight string
+
 ---@class (exact) render.md.UserHtml: render.md.UserBaseComponent
 ---@field public comment? render.md.UserHtmlComment
+---@field public tag? table<string, render.md.HtmlTag>
 
 ---@class (exact) render.md.UserLatex: render.md.UserBaseComponent
 ---@field public converter? string
@@ -799,6 +804,11 @@ M.default_config = {
             -- Highlight for the inlined text
             highlight = 'RenderMarkdownHtmlComment',
         },
+        -- HTML tags whose start and end will be hidden and icon shown
+        --   The key is matched against the tag name
+        --   'icon':      Gets inlined at the start
+        --   'highlight': Highlight for the icon
+        tag = {},
     },
     -- Window options to use that change between rendered and raw view
     win_options = {

lua/render-markdown/render/html_tag.lua

@@ -0,0 +1,42 @@
+local Base = require('render-markdown.render.base')
+
+---@class render.md.render.HtmlTag: render.md.Renderer
+---@field private tag render.md.HtmlTag
+local Render = setmetatable({}, Base)
+Render.__index = Render
+
+---@return boolean
+function Render:setup()
+    local tag = self.node:child('start_tag')
+    if tag == nil then
+        return false
+    end
+    local name = tag:child('tag_name')
+    if name == nil then
+        return false
+    end
+    self.tag = self.config.html.tag[name.text]
+    return self.tag ~= nil
+end
+
+function Render:render()
+    self:hide('start_tag')
+    self:hide('end_tag')
+    self.marks:add(false, self.node.start_row, self.node.start_col, {
+        virt_text = { { self.tag.icon, self.tag.highlight } },
+        virt_text_pos = 'inline',
+    })
+end
+
+---@private
+---@param child string
+function Render:hide(child)
+    local node = self.node:child(child)
+    if node ~= nil then
+        self.marks:add_over(true, node, {
+            conceal = '',
+        })
+    end
+end
+
+return Render

lua/render-markdown/state.lua

@@ -300,6 +300,11 @@ function M.validate()
                             :type('text', { 'string', 'nil' })
                             :check()
                     end)
+                    :nested('tag', function(tags)
+                        tags:nested('ALL', function(tag)
+                            tag:type({ 'icon', 'highlight' }, 'string'):check()
+                        end, false):check()
+                    end)
                     :check()
             end)
             :nested('win_options', function(win_options)

lua/render-markdown/types.lua

@@ -23,6 +23,7 @@
 
 ---@class (exact) render.md.Html: render.md.BaseComponent
 ---@field public comment render.md.HtmlComment
+---@field public tag table<string, render.md.HtmlTag>
 
 ---@class (exact) render.md.Latex: render.md.BaseComponent
 ---@field public converter string