Allow text search providers to show the maxResults indicator after a custom number of results

[roblourens]

Some extensions like Live Share would like to show the yellow maxResults hint in the search view after a lower number of results than our max of 10000.

This could look like

interface ITextSearchResult {
  hitLimit?: boolean;
}

export interface TextSearchProvider {
    provideTextSearchResults(query: TextSearchQuery, options: TextSearchOptions, progress: Progress<TextSearchResult>, token: CancellationToken): Thenable<ITextSearchResult>;
}