optimize dynamic regex matching

Summary:
allow such matches to be cached, too, by guarding the cache
with the last used captures

makes VHDL highlighting instantanious for bug 418778

BUG: 418778

Test Plan:
make && make test

example from bug 418778 is fast!

Reviewers: dhaumann, vkrause, nibags

Reviewed By: dhaumann

Subscribers: mwolff, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D28039