fix: wrap nvim-cmp register_source in pcall

## Details

Issue: #298

Unsure how exactly this can happen but apparently using `pcall(require, 'cmp')`
on its own is not enough to check that the user has `nvim-cmp` installed
with all associated methods. Fix is easy enough and wraps the `register_source`
call in a pcall.

Author: MeanderingProgrammer

doc/render-markdown.txt

@@ -1,4 +1,4 @@
-*render-markdown.txt*          For 0.10.0         Last change: 2025 January 14
+*render-markdown.txt*          For 0.10.0         Last change: 2025 January 15
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*

lua/render-markdown/health.lua

@@ -5,7 +5,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '7.8.7'
+M.version = '7.8.8'
 
 function M.check()
     M.start('version')

lua/render-markdown/integ/cmp.lua

@@ -35,7 +35,7 @@ function M.setup()
     if not has_cmp then
         return
     end
-    cmp.register_source(Source:get_debug_name(), Source)
+    pcall(cmp.register_source, Source:get_debug_name(), Source)
 end
 
 return M