fix(parser): forToken cache leak

[peaklabs-dev]

Issue

Parser::$cache was static, so sub-parsers created by forToken() were shared across every Parser instance in the process. The first instance to parse a given token type "won" and its configuration was baked into the cached clone and silently reused by all later instances, regardless of how they were configured.

$first = new Parser();
$first->parse('`x`');

$second = new Parser(highlighter: null);
$second->parse('`<b>x</b>`')->html;
// before: rendered by the cached sub-parser from $first, using its highlighter
// after: rendered with $second's own configuration

Once there are more configuration options like security settings new Markdown(allowedUrlPrefixes: []) and so on this becomes a way bigger issue.

Changes

• Scope forToken cache per instance
• Add cache leak regression test