Add slashit functions extension & fix stripslashes extension

[IanDelMar]

Adds a dynamic function return type extension for handling:

1. untrailingslashit() — removes trailing forward slashes and backslashes,
2. trailingslashit() — adds a trailing slash after removing any trailing slashes,
3. backslashit() — adds backslashes before letters and before a number at the start of a string.

For (1) and (2), the extension uses the types PHPStan provides for rtrim() and string concatenation.
For (3), the extension traverses types, generalising constant strings to strings with accessory types, and replaces numeric-string (backslashes will be added) and non-empty-string (if it were '0', backslashes would be added) with non-falsy-string.

Fixes the StripslashesFromStringsOnlyDynamicFunctionReturnTypeExtension by letting PHPStan handle the type for non-constant string arguments (e.g. a non-falsy string might be '\\', which becomes ''). Currently, PHPStan returns string for calls to stripslashes(). If PHPStan ever decides that stripslashes() should be, or is worth being, handled more granularly, this extension will align with that.

[szepeviktor]

It fails with Locking phpstan/phpstan (2.0.0)
How high should I raise it?

phpstan-wordpress/composer.json

Line 16 in 49b833d

"phpstan/phpstan": "^2.0"

[IanDelMar]

Since 2.1.18, there are no errors; the "errors" (nothing is incorrect--prior to 2.1.18 the types were simply wider) occurred because PHPStan improved its inference of rtrim()’s return type. It’s unfortunate that we have to bump the version to overcome this.

[szepeviktor]

Done.