Maintain sourcemap file extensions

[tagCincy]

The regular expression for generating digested paths in propshaft is updated to accommodate sourcemap files. This will ensure the digested sourcemaps end -sha.ext.map instead of ext-sha.map. Services like Datadog that allow sourcemap uploads look specifically for files ending with .js.map, so this change allows compatibility. A corresponding test has been added in asset_test.rb, verifying that sourcemap files are correctly digesting their paths.

[brenogazzola]

Looks good, can you resolve the conflicts Github is complaning so we can merge please?