Removing autocomplete triggers breaks autocomplete

[cameron-martin]

4be1b3bbf38dbddfca9aabacbff82a644681ae9d removes some autocomplete triggers. However, this breaks autocomplete for labels in vscode.

Before this commit, I could hit ctrl+space in a string and get autocomplete suggestions from the language server:

However, after this change I only get the default vscode autocomplete:

[cameron-martin]

I believe the issue here is we're now sending None as completion_provider, which tells the client that the server does not support autocomplete. Instead it should be something like Some(CompletionOptions::default()).