Feed creation refactoring

This breaks up the humongous functions from feed.php into multiple
classes. To keep compatibility with existing Plugin events, the basic
principle of how the feed is assemb

Feed creation refactoring

This breaks up the humongous functions from feed.php into multiple
classes. To keep compatibility with existing Plugin events, the basic
principle of how the feed is assembled has not been changed:

* depending on the given mode and other options lose arrays of items are
gathered
* these items are then converted (again based on the various options)
into proper FeedItems
* the FeedItems are then added to the Feed

The conversion from loosely typed item data to something more structured
is now done by the FeedItemProcessor classes. Some very basic tests have
been added. It does not cover erverything but covers more than before
(which was nothing).

Manual testing before merging this is highly recommended. I am not
confident that I ported over everything correctly.

No new features have been added, but especially media support could and
should be improved in the future.

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Use str_starts_with/str_ends_with

code style: operator spacing

coding style: control flow whitespaces

Apply rector fixes to inc/File

import classes, replace dbglog, simplify, remove unused statements, and other warnings from IntelliJ

update phpdocs,
rename dokuwiki/Ui/Draft to PageDraft

rename namespace Media to File

This is a bit more generic and allows us to have additional File
abstractions in here later.