autocomplete and import vals

[jrmann100]

For parity with the val.town editor, adds:

• A completions hook which calls the autocomplete API.
• A command which imports a val from a completion.
• Customization options for versioning behavior when (re)importing vals.

Please test:

• Manually triggering completions with ctrl + space
• Automatically triggering completions on @[<handle>[/<val>]]
• Importing default exports and named exports
• Importing a val which has not been imported before
• Importing a val which has been imported before (with a different version? with other exports?)
• Other combinations of the above
• Settings for auto import

[jrmann100]

Starting to think that this should be true by default for parity with the editor.

[pomdtr]

Hey, thanks a lot for your contribution, it looks incredible! I took a quick look at the code (looks good), and I'll test the extension on my instance of vscode tomorrow.

I wonder if all these customization options are necessary. I would vote for just following whatever behaviour the website editor has, so the end user doesn't get confused.