Report on actionable emit hotspots

[amcasey]

After looking at lots of traces it appears that JS emit is nearly always proportional (with a small constant) to file size (even with down-levelling), so flagging JS files that took a long time to emit rarely does anything beyond identifying large input files.

Declaration emit, on the other hand, is much less directly related to input size. So far, the most important factor we've found for disproportionately slow declaration emit is the presence of a lot of import expressions in the output. This PR attempts to flag such instances by searching slow-to-emit declaration files (if present) for import expressions.

Fixes #3 (until we find some other cause of disproportionate slowdown).

[amcasey]

If this functionality helps you, please upvote microsoft/TypeScript#44044 and, if possible, provide a repro.