[compressAssets] Avoid double-compression

[znerd]

Currently the compressAssets rule indicates that I should compress WOFF2 files. Since WOFF2 files are already compressed, I don't think that is appropriate advice.

In fact I would suggest inverting the rule, and validate that already compressed files (WOFF2, GZIP, to name a few) are indeed not compressed again.

Context:

• SiteSpeed.io 7.4.0 (with Coach 2.2.0)

[soulgalore]

Hi @znerd yeah. Do you have time to do a PR? I'm planning to rework the advice late October or late November, but before that I will focus on getting the next sitespeed.io release out there.

Best
Peter