Add search provider for scheme vscode-test-web

[wkillerud]

Hello 👋

I've investigated the file search provider proposal in order to solve #39

Unfortunately, actually filtering seems to be blocked by microsoft/vscode#200892 (edit: only on the provider side, filtering happens on the client)

I'm testing with https://github.com/wkillerud/vscode-scss/tree/repro/no-search-provider and npm linking in ./web. The language server now gets a list of files in the workspace, where before it would get nothing.

[wkillerud]

This seems to be working pretty well, testing with https://github.com/wkillerud/vscode-scss/tree/repro/no-search-provider

I noted over on microsoft/vscode#200892 (comment)

It looks like the filtering is done correctly on the client-side / in VS Code, so the provider can return all files as "matches" and still kind of work.

I'll mark this as ready for review.

[aeschli]

Thanks @wkillerud !