[PHP 8.4] Add mb_ucfirst() and mb_lcfirst()

[zonuexe]

resolve #470

This implementation is also tested and benchmarked in my repository: https://github.com/zonuexe/polyfill-mb_ulcfirst

[zonuexe]

These errors below looks strange:

40) Warning
Incompatible signature for PHP >= 8:
- mb_ucfirst(string $string, ?string $encoding = null): string
+ mb_ucfirst(string $string, ?string $encoding = null): string

41) Warning
Incompatible signature for PHP >= 8:
- mb_lcfirst(string $string, ?string $encoding = null): string
+ mb_lcfirst(string $string, ?string $encoding = null): string

Internal functions up to PHP 8.0 implicitly accepted null as a string. It seems that the type is declared as ?string instead of string for compatibility in polyfills up to php80.

polyfill/src/Util/TestListenerTrait.php

Lines 126 to 136 in df4793d

	$map = [
	'?' => '',
	'IDNA_DEFAULT' => \PHP_VERSION_ID >= 80100 ? 'IDNA_DEFAULT' : '0',
	'array|string|null $string' => 'array|string $string',
	'array|string|null $from_encoding = null' => 'array|string|null $from_encoding = null',
	'array|string|null $from_encoding' => 'array|string $from_encoding',
	];
	if (strtr($polyfillSignature, $map) !== $originalSignature) {
	$warnings[] = TestListener::warning("Incompatible signature for PHP >= 8:\n- {$f['name']}$originalSignature\n+ {$f['name']}$polyfillSignature");
	}

In the code below, ? is removed to check the difference between the internal function and the polyfill interface.

PHP 8.4 deprecates implicit nullable, so this test should be replaced with more stringent checks.

• PHP: rfc:deprecate-implicitly-nullable-types

[zonuexe]

This test was affected by changes in the native mb_decode_numericentity() function.

https://3v4l.org/fpGFH

It seems that other tests also need to call polyfill methods instead of built-in function.

[Ayesh]

Possibly duplicate of #466

[zonuexe]

@Ayesh Sorry, I missed that PR because the issue didn't exist. It looks like my patch is covered by your implementation.

[Ayesh]

Let's put the efforts together :)
If you see anything that we could improve in the other PR, or if you think merging this PR is the way to go, please do comment.