[TASK] Mark parsing-internal classes and methods as @internal (#674)

Code that uses this library is not expected to call internal parsing
functionality. Communicate this with the corresponding `@internal`
annotation.

This allows us to boldly refactor the parser code.

Part of #668

Author: oliverklee

CHANGELOG.md

@@ -15,6 +15,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Changed
 
+- Mark parsing-internal classes and methods as `@internal` (#674)
 - Block installations on unsupported higher PHP versions (#691)
 - Improve performance of `Value::parseValue` with many delimiters by refactoring
   to remove `array_search()` (#413)

src/Parsing/Anchor.php

@@ -4,6 +4,9 @@
 
 namespace Sabberworm\CSS\Parsing;
 
+/**
+ * @internal
+ */
 class Anchor
 {
     /**

src/Parsing/ParserState.php

@@ -7,6 +7,9 @@
 use Sabberworm\CSS\Comment\Comment;
 use Sabberworm\CSS\Settings;
 
+/**
+ * @internal
+ */
 class ParserState
 {
     /**