Provide passive `extendedDiagnostics` feedback on PRs

[DanielRosenwasser]

@amcasey had suggested that we provide performance feedback for changes, similar to what we do on DefinitelyTyped. This could be a part of the standard CI process, where @typescript-bot provides the output of tsc -p src/compiler as a comment and compares it against a baseline.

[DanielRosenwasser]

This would be a nice to have, but we might not get around to it in the 3.8 timeframe.