Migrate SKIPIF -> EXTENSIONS (#7138)

This is an automated migration of most SKIPIF extension_loaded checks.

Update mbstring parameter names

Closes GH-6207.

Promote unknown encoding throws in encoding array/string list

For the string list we emit still emit a warning by comparing arg_num to 0

Closes GH-5337

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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Parse mb_convert_encoding() encodings only once

Instead of re-parsing them for every converted value. Also reuse
the generic parse_array() helper.

Properly report unknown encoding in encoding lists

And clean up the related array and list parsing code.

Fix length inconsistency in mb_convert_encoding

Don't mix strlen() and ZSTR_LEN(). If the encoding contains a
NULL byte, this will overflow the buffer.

NULL bytes will still mak

Fix length inconsistency in mb_convert_encoding

Don't mix strlen() and ZSTR_LEN(). If the encoding contains a
NULL byte, this will overflow the buffer.

NULL bytes will still make this behave oddly because the consuming
code will cut off the string there, but let's address that in master...

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Fix test expectation

As of PHP 8.0.0, array to string conversion raises a warning.

Fix #79149: SEGV in mb_convert_encoding with non-string encodings

We must not assume that `hash_entry` `IS_STRING`, but rather use
`encoding_str` which is guaranteed to be.