Skip compression of already compressed files

[PetrDlouhy]

New take on #296

[lapinvert]

@evansd Any updates on this?

[PetrDlouhy]

@lapinvert @adamchainz I have rebased this to the current main, but I am not sure if there is interest in getting this merged.
I expect that skipping already generated files (named with MD5 sums) would be only beneficial and speed things up for most people. But I can make this configurable (as default or non-default).

Also this would be very easily achieved when #354 is merged by simple Compressor override.

[lapinvert]

To me it sounds a no-brainer feature to have. For projects with many static files it makes a huge difference.