extension: use streaming search for suggestions

Created by: novoselrok

We are moving away from GQL suggestions towards using our streaming search as a suggestions source. See base PR: https://github.com/sourcegraph/sourcegraph/pull/25941

It also turns out that in the current extension the suggestions do not appear. So I made a small change to get them working again. For context: the requestGraphQL function was undefined when trying to fetch suggestions from the API. This line: private suggestionFetcher = createSuggestionFetcher(20, this.requestGraphQL) should have been moved to the constructor where this.requestGraphQL would have been defined. By using the streaming search suggestions we sidestep the issue.

Fixes #780 (closed)