[Php80] Skip non-zero mb_strpos() offset in MbStrContainsRector

[guillaume-sainthillier]

Same as #8640, for MbStrContainsRector: an offset mb_strpos() is rewritten to str_contains(mb_substr($haystack, $offset), $needle), which copies the haystack on every call. mb_substr() also has to walk the string to count characters, so it's much worse than substr().

1,000 calls on a 1 MB string, offset 1000, PHP 8.5:

case	mb_strpos(..., $offset)	str_contains(mb_substr(...))
needle ~1 KB after offset	0.5 ms	1,217 ms
needle absent	21 ms	1,279 ms

Change:

• offset resolving to 0: str_contains($haystack, $needle), no mb_substr()
• any other offset: skipped

[TomasVotruba]

Valid change, thank you 👍