[TASK] Mark the OutputFormat as not extendable

This is the V8.x backport of #1131.

Author: oliverklee

CHANGELOG.md

@@ -11,6 +11,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Changed
 
+- Mark the `OutputFormat` the constructor as `@internal` (#1133)
 - Mark `OutputFormatter` as `@internal` (#896)
 - Mark `Selector::isValid()` as `@internal` (#1037)
 - Mark parsing-related methods of most CSS elements as `@internal` (#908)
@@ -19,6 +20,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Deprecated
 
+- Deprecate extending `OutputFormat` (#1133)
 - Deprecate `OutputFormat::get()` and `::set()` (#1107)
 - Deprecate support for `-webkit-calc` and `-moz-calc` (#1086)
 - Deprecate `__toString()` (#1006)

src/OutputFormat.php

@@ -3,7 +3,7 @@
 namespace Sabberworm\CSS;
 
 /**
- * Class OutputFormat
+ * Extending this class is deprecated in version 8.8.0; it will be made `final` in version 9.0.0.
  *
  * @method OutputFormat setSemicolonAfterLastRule(bool $bSemicolonAfterLastRule) Set whether semicolons are added after
  *     last rule.
@@ -239,6 +239,9 @@ class OutputFormat
      */
     private $iIndentationLevel = 0;
 
+    /**
+     * @internal since V8.8.0. Use the factory methods `create()`, `createCompact()`, or `createPretty()` instead.
+     */
     public function __construct()
     {
     }