mediawiki.inspect: add CSS report

This patch extends mediawiki#inspect to add a notion of 'reports'. Instead of
there being one single view for all ResourceLoader module debug data, you can
now request that a specific report be run. Each report highlights a set of
metrics that are unified by some common theme.

The previous, unqualified report is now branded the 'size' report. In addition
to it, this patch also adds a CSS report, which prints the count of selectors
and the count of selectors that match against the current time

Change-Id: I0e7a47b5a93a66ecb103fe30a3de8a2ca8a4eb84
(cherry picked from commit 678d976f0b646c98ca679f0dbb47602da72a947c)