Add support for preloading files in nested directories

[jasalt]

Allow PhpWeb constructor's files parameter and tag attribute data-files to preload files with nested directory structure.

This way e.g. Composer's vendor directory can be preloaded by specifying it's content as a single JSON object / file which can be created with script such as https://gist.github.com/jasalt/b50f60914be75b8dd3ee18f878d801c1.

Provides alternate way to initializing WASM FS where currently copying a single compressed file and extracting it in WASM PHP code is one popular method. This does not require running any PHP initialization code in Wasm.

[jasalt]

Didn't test building this yet, just patched this code locally in NPM package sources and got with Composer require_once '/vendor/autoload.php'; working with it (demo repo).