improve performance of browser ext completion
Created by: sqs
- create less garbage in chromePortListener
- defer UI updates until the animation frame
- better throttling of EditorCompletionWidget calls to the providers in response to textarea changes
Partially addresses https://github.com/sourcegraph/sourcegraph/issues/3433.
These signiicantly improve the perf problem to the point where it feels as smooth as native GitHub to me.