Add regex filtering

[imwints]

Filters starting with '!' will try to match processes pid, name, cmd and user with extended regex as defined by the C++ standard. A single '!' will not filter.

Partially implements https://github.com/aristocratos/btop/issues/387, but is missing the option to filter by type, meaning by user or command only.

I'm also thinking about adding some tests to verify filtering works as expected, I managed to break the filtering while implementing this 🤣. And I also see more opportunity to share code from all platforms.