refactor(preg): return the preg_* $matches instead of filling an out-param

PHP fills `$matches` through a by-ref parameter, which the port mirrored
with a `&mut` out-param plus a bool or count return. Every caller then
had to declare an empty binding one line ahead of the call, and nothing
in the type said the binding is only meaningful when the call succeeded.
Return the matches instead: preg_match() and preg_match2() hand back an
Option, and the three preg_match_all* functions hand back the collection
they used to fill.

The occurrence count the two map-shaped preg_match_all* functions used
to return is the length of any one of the map's columns, so it is not
lost -- Preg::match_all() and friends derive it via occurrence_count().

preg_replace2() keeps its `count: Option<&mut usize>`: that one is not
derivable from the replaced string, and callers that do not want it pay
nothing for passing None.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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