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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replace magic strings with class constants in Lexer

Introduce MODE_EXIT and MODE_SPECIAL_PREFIX constants to replace
the undocumented "__exit" and "_" string conventions used for
mode transitions.

Use str_starts_with/str_ends_with

code style: line breaks

code style: operator spacing

Apply rector fixes to inc/Parsing

Update core code to make use of sexplode()

This makes use of our own explode mechanism everywhere were we expect a
fixed number of results.

Move defines to their own file

As described in
https://github.com/dwp-forge/columns/issues/5#issuecomment-638467603
sometime the Lexer constants have not been (auto)loaded when a syntax plugin
is in

Move defines to their own file

As described in
https://github.com/dwp-forge/columns/issues/5#issuecomment-638467603
sometime the Lexer constants have not been (auto)loaded when a syntax plugin
is invoked (I'm not sure why).

In general PSR2 discourages a mix of main code and function/class setup
with the call to define() being considered main code.

This patch moves these the define calls to a separate new file, solving
both of the above problems.

These are not all our defines. Instead I focused on the ones that are
ENUM-like.

In the future we should think about what defines can be replaced by
class constants and what other define() calls should be moved.

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Let plugins access the lexer mode stack

Make lexer/state stack more understandable
- rename lexer $mode property to avoid two different uses of "mode"
variables in the lexer
- clarify/improve comments

moved all parsing related namespaces to their own