Better integration with local filesystem for web REPL

[jack-arms]

It's nice to have files automatically saved under "patterns -> user", but I generally prefer to keep files locally so that I can manage them with Git for example. There are import/export options, so that you can import a local file, but export only exports all files in a single JSON file, so when working with a single file I think copy/pasting back and forth is still the best option.

With the File System Access API (see demo: https://mburakerman.github.io/file-system-access-api-demo/) I think it should be possible to import a local file, edit it as you would for any other Strudel code, and then have a save button that uses the API to write back to the local file.