Support imports in VS Code

[berekuk]

This should be done after #1331.

[berekuk]

This is complicated.

Playground webview renders current text in the editor, even unsaved text, so it can't read files from disk (even if it could; webviews can't read files anyway, and any content should be sent to webviews with events).

But with imports, playground should also get contents of imported files somehow, and also update when the files have changed.

Preliminary plan:

• webview will ask the extension for all dependencies it parsed from code, with events
• extension will send events with the content for dependencies (read from disk), which the playground will add to its project object
• extension will also monitor changes to the files (with onDidChangeTextDocument and so on) and send updated versions to the playground (but only the files that are needed by the playground, so we should keep track of those somehow; we don't want to send all edited code to every squiggle webview panel)
• async handling of all this is going to be quite messy, since the playground shouldn't render errors if files are not loaded yet