* changed the way we report live analysis to task center

previously, we listen to diagnostic service to report progress. problem is that, it only raise event if it found errors on a file. so what we report is actually last file we found errors on rather than file that we are analyzing.

this caused confusion since we report in task center that we are analyzing file "A" when it is actually "analyzed" not "analyzing"

another issue is since it only report file that contains errors. we might not actually show anything in task center if there is no error, or show file "A" for long time if that is only file with errors.

this PR changes the experience closer to what users would expects. and now progress is for solution crawler not specifically on diagnostics.

now we report file that solution crawler is analyzing.

there is still cavet such as solution cralwer can pause between processing a file if VS is busy. but it will still show file "A". or we will not update UI at least 200ms a part and etc.

since it is task center where we don't want to be too impactful to VS, based on feeedback we will see whether we need to do more such as detect solution crawlwer pause and update task center to show pasue. or update task center to show different stage such as analyzing/analyzed.

or show in task center, what analyzer is actually running such as diagnostic, todo, designer attribute scan, find all reference cache and etc.

* addressing PR feedbacks