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 mb_str_split warning to ValueError

Also add a TODO about documenting this funcion on PHP.net
Convert some checks to assertions as if they don't hold something went wrong during m

Promote mb_str_split warning to ValueError

Also add a TODO about documenting this funcion on PHP.net
Convert some checks to assertions as if they don't hold something went wrong during memory allocation
Due to these changes this function cannot return false anymore, fix stubs accordingly

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