✨ New FilePath utility class

[jrfnl]

... with some utility functions for handling file paths.

The class initially contains the following utility methods:

• getName(File $phpcsFile): string - Retrieve the normalized file name for the current file.
• isStdin(File $phpcsFile): bool - Check whether the current file under scan comes from STDIN.
• normalizeAbsolutePath(string $path): string - Normalize an absolute path to forward slashes and to include a trailing slash for directories.
• normalizeDirectorySeparators(string $path): string - Normalize all directory separators to be a forward slash.
• trailingSlashIt(string $path): string - Ensure that a directory path ends on a trailing slash.
• startsWith(string $haystack, string $needle): bool - Check whether one file/directory path starts with another path.

Includes unit tests.

[jrfnl]

Rebased without changes. Setting to draft as this PR depends on #550, which has to be merged first.

[jrfnl]

Rebased without changes after the merge of PR #550.

[jrfnl]

Rebased without changes.
Marking as ready as #550 has been merged now. Merging once the build passes.

Note: I've looked the sniff over to check if any of the new exceptions from PR #598 should be implemented and concluded that things should be fine as-is. In all methods, it is either handled gracefully already or a param will be passed to a PHP native function, which may throw a type error. I think that should be sufficient.