Adds a bundlesize test that measures the bytesize of
individual ResourceLoader modules.
This test depends on a running mediawiki instance as well as
the $MW_SERVER and $MW_SCRIPT_PATH environment variables.
ResourceLoader modules are fetched from a URL and piped
into a bundlesize command based on a custom configuration file.
The test can be run with `npm run test:size`.
Bug: T244276
Change-Id: I4f4534921ccbc6bd4f99229c8dd36b1279dde640
Jan Drewniak