Support compression of non-aggregated assets

The static compression logic is very closely tied to aggregation. However ✨ [PP-1] Allow setting aggregation groups for js files in library definitions Postponed would allow libraries to specify that only files from that library should be aggregated together, and that would achieve more or less the same thing I think.

If files aren't aggregated and aren't in a writable directory then there'd be no way to write compressed versions to that directory.

Going to postpone on that issue as the most likely way to implement this.

That will also help.

The idea here is that it can also be useful for modules to push assets that only need compression and not limited to js and css.

Modules such as Matomo, Svg Image, and Responsive Favicons then can use that logic.