Remove unnecessary mbstring skipifs

These functions are always available (if the extension is available
at all).

Migrate more SKIPIF -> EXTENSIONS (#7139)

This is a mix of more automated and manual migration. It should remove all applicable extension_loaded() checks outside of skipif.inc files.

Update mbstring parameter names

Closes GH-6207.

Promote some warnings to ValueError in mbstring

Promoted warnings are:
* Empty encoding lists
* Unknown language
* Start and Width out of bound