Add configuration for running etsy/phan against core

Add configuration for running etsy/phan against core

Adds the necessary configuration and stubs for phan static analysis to
run against mediawiki core. A variety of fixes have been applied to core
recently such with this current configuration we are completely passing,
except for one issue with a bug in Phan (https://github.com/etsy/phan/issues/286)

In it's current configuration Phan will detect fatal errors trying to
access non existant classes, undefined method calls, and other errors.
The analysis can be expanded as we cleanup more of the codebase.  This
is in preparation for working on getting the CI to run phan as part of
the review process. I have found phan to be usefull in CirrusSearch
(although with stricter config) for catching bugs and I think it could
help in core as well.

We arn't too far from being able to disable `null_casts_as_any_type`,
there are only a few classes that need adjusting. Lots more work needs
to be done to reduce `minimum_severity` from 10 to 5 (normal) or
0 (low).

Change-Id: Iafd55b1380f37d7b0d195eed081f8042166690e8