fix (parsing): avoid newline loss on GFM section editing

The GFM header parsing returned a byte position pointing at the newline
before the actual header resulting in the observed newline eatings as

fix (parsing): avoid newline loss on GFM section editing

The GFM header parsing returned a byte position pointing at the newline
before the actual header resulting in the observed newline eatings as
reported in https://github.com/dokuwiki/dokuwiki/pull/4636#issuecomment-4491970909

Additionally this fixes an oddity of DW header parsing which
accidentally allowed text on the line before the opening = chars.
Whitespace is still allowed.

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move handler methods into ParserMode classes and rename Handler

Each ParserMode class now implements handle() from ModeInterface,
containing the token handling logic that previously lived as individ

move handler methods into ParserMode classes and rename Handler

Each ParserMode class now implements handle() from ModeInterface,
containing the token handling logic that previously lived as individual
methods on Doku_Handler.

The Handler class (formerly Doku_Handler) is the single dispatch point:
Lexer passes tokens to Handler::handleToken() which routes to mode
objects, plugins, or returns false. The Lexer only tokenizes and
resolves mapHandler aliases.

Key changes:
- Add handle() to ModeInterface, implemented by all mode classes
- Move Doku_Handler to dokuwiki\Parsing\Handler namespace
- File extends Code (shared parsing via $type property)
- Quotes uses mapHandler() + Handler::getModeName() for sub-modes
- Media::parseMedia() replaces Doku_Handler_Parse_Media()
- Code::parseHighlightOptions() replaces parse_highlight_options()
- Per-parse state (footnote, doublequote) stays on Handler
- Deprecated wrappers kept for base/header/internallink/media
- Class alias and rector rules added for backward compatibility

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code style: line breaks

moved all parsing related namespaces to their own