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.

Only force log startup errors if display_startup_errors disabled

Otherwise this results in duplicate errors.

Closes GH-5941.

Throw warning if invalid internal_encoding ini is specified

Convert some unknown encoding warnings to ValueErrors in ext/mbstring

Promotes only the warnings where the encoding comes only from a string.
Functions which accept an array of encodings

Convert some unknown encoding warnings to ValueErrors in ext/mbstring

Promotes only the warnings where the encoding comes only from a string.
Functions which accept an array of encodings will be fixed at a later stage.

Closes GH-5317

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