feat: better association of .js to Source

[heyzec]

Typically, VS Code knows which LSP server to launch as the filetype is based on the extension of the file. We previously discussed and agreed that we do not want to create a custom file extension, e.g. .sjs, to represent Source.

To workaround this limitation, this PR uses to approaches:

1. Extension modifies workspace-specific settings.json for Source Academy folder, such that .js files here will now be associated with Source.
2. Regardless of which workspace the user's VS Code is in, editor buffers created by the extension will be programmatically set to Source.

[heyzec]

Thanks @mug1wara26 for testing!