Add and extract `SymlinkFileSystem` package

[BrianHenryIE]

There are currently some file operations that check for symlinks.

Give the filesystem abstraction of FlySystem, including ReadOnlyFileSystem, I think synlinks should be handled in their own proxy layer.

By extracting the class as its own package, it will make it easier to unit test, particularly for Windows users to test and contribute without needing to think of the broader project

strauss/src/Pipeline/Cleanup.php

Lines 88 to 99 in df9c86a

	if (false !== strpos('WIN', PHP_OS)) {
	/**
	* `unlink()` will not work on Windows. `rmdir()` will not work if there are files in the directory.
	* "On windows, take care that `is_link()` returns false for Junctions."
	*
	* @see https://www.php.net/manual/en/function.is-link.php#113263
	* @see https://stackoverflow.com/a/18262809/336146
	*/
	rmdir($symlinkPath);
	} else {
	unlink($symlinkPath);
	}

See: #64