Extension FS that crosses the webview/host boundary

[DavidSouther]

When compiling a chip, if a part isn't builtin, the loader will attempt to read that chip's .hdl from the current directory. This requires a FileSystem, which is fine for tests, web, and CLI. The extension needs a FileSystem implementation that can request, from the webview, a certain hdl file and receive it at a later time.

[DavidSouther]

Closing, as we aren't pursuing a VSCode extension. Maybe re-open if we reconsider.