VS Code becomes unresponsive for silly searches

[weinand]

Testing #50498:

• open vscode workspace with VS Code
• search for 'e'

Observe:
VS Code becomes unresponsive and the cancel button appears very late (if at all), so there is almost no chance to cancel a search.

[roblourens]

Yes, with the original search, the frontend gets complete FileMatches. With the new one, it gets a stream of individual matches which have to be merged into FileMatches. The way I do this right now is inefficient and we spend a ton of time there...