Add unit tests for mb_str_split/mb_substr on MacJapanese encoding

MacJapanese has a somewhat unusual feature that when mapped to
Unicode, many characters map to sequences of several code

Add unit tests for mb_str_split/mb_substr on MacJapanese encoding

MacJapanese has a somewhat unusual feature that when mapped to
Unicode, many characters map to sequences of several codepoints.
Add test cases demonstrating how mb_str_split and mb_substr behave in
this situation.

When adding these tests, I found the behavior of mb_substr was wrong
due to an inconsistency between the string "length" as measured by
mb_strlen and the number of native MacJapanese characters which
mb_substr would count when iterating over the string using the
mblen_table. This has been fixed.

I believe that mb_strstr will also return wrong results in some cases
for MacJapanese. I still need to come up with unit tests which
demonstrate the problem and figure out how to fix it.

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Correct entry for 0x80,0xFD-FF in SJIS multi-byte character length table

As a performance optimization, mbstring implements some functions using
tables which give the (byte) length of a

Correct entry for 0x80,0xFD-FF in SJIS multi-byte character length table

As a performance optimization, mbstring implements some functions using
tables which give the (byte) length of a multi-byte character using a
lookup based on the value of the first byte. These tables are called
`mblen_table`.

For many years, the mblen_table for SJIS has had '2' in position 0x80.
That is wrong; it should have been '1'. Reasons:

For SJIS, SJIS-2004, and mobile variants of SJIS, 0x80 has never been
treated as the first byte of a 2-byte character. It has always been
treated as a single erroneous byte. On the other hand, 0x80 is a valid
character in MacJapanese... but a 1-byte character, not a 2-byte one.

The same applies to bytes 0xFD-FF; these are 1-byte characters in
MacJapanese, and in other SJIS variants, they are not valid (as the
first byte of a character).

Thanks to the GitHub user 'youkidearitai' for finding this problem.

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Fix mb_str_split on empty strings in variable-length text encodings

Previously, when passed an empty string, and given an encoding which
uses a variable number of bytes per character (an

Fix mb_str_split on empty strings in variable-length text encodings

Previously, when passed an empty string, and given an encoding which
uses a variable number of bytes per character (and which doesn't have
a 'character length table'), mb_str_split would return an array
containing a single empty string, rather than an empty array.

The ISO-2022 encodings are among those which were affected by this bug.

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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.

Remove stray mentions of mbstring.func_overload

This feature has been completely removed.

Closes GH-6688.

Implement mb_str_split()

RFC: https://wiki.php.net/rfc/mb_str_split