Support for TabCompletion

[jrieken]

We have a first cut for the 'tab completion'-feature which is about inserting a snippet when matching a prefix and pressing Tab.

Open questions:

• by default we have '24x7' Intellisense. combined with the new snippetSuggestions = top setting you can achieve the same
• for Tab to work we need to know snippets statically which isn't true for completions of type snippet that come from contributed completion providers (IMO this is a corner case)

[jrieken]

ghetto tab completion using the snippetSuggestions property

[jrieken]

@Microsoft/vscode looking for feedback on this

[d4rkr00t]

I've tested it a bit. And it works great, thanks so much!