Having options to remove certain files from the asset map

Hi,

I found the add-on super helpful for the project that I'm working on. One improvement that I think would be very nice to have is to provide users with an option to ignore certain files in the asset map so that they only have to keep track of the mappings for assets that they really need.

I'm imagining having some of these options available:
- `excludeExtensions`: an array of file extensions that will be ignored. In my project we don't care about mappings of `.js`, `.css`, or `.map` files.
- `filter`: a function that will be called on each mapping in the asset map. If it returns `true` for a key-value pair, the pair will be kept. Otherwise the pair will be removed from the map.

Once again, thanks for the awesome add-on!

No files, tests, or entry points are named. Start by locating the add-on’s asset-map generation and configuration entry points, then determine how extension exclusions and a mapping filter should be exposed. Done means the asset map omits excluded extensions and pairs rejected by the filter, with the behavior documented and tested.

Written by the indexing model from the issue text.