Add mediawiki.inspect module

This patch adds a mediawiki.inspect ResourceLoader module. The module contains
tools for inspecting page composition and performance. It is currently
restricted to a set of static methods for querying the ResourceLoader module
registry, the most useful of which is #inspectLoadedModules, which prints a
list of loaded ResourceLoader modules and their size in kilobytes, ordered from
largest to smallest. This method has a lazy-loading alias: mw.loader.inspect.

Change-Id: I5a007fdf11196664f425cd7800eec7fc3fdb629f
(cherry picked from commit 0afecbaf07e6b540f3c09ffa83ddd9030ba3ec07)