Add the mediawiki.experiments module

The module provides a generic bucketing function - it accepts an
experiment specification and a token that identifies a unique user - and
doesn't have any side effects, i.e. the bucket isn't persisted to
storage. It is therefore assumed that clients are responsible for either
storing the token or storing the bucket for the duration of an
experiment.

The module was extracted from the - admittedly, unused - module of the
same name in the MobileFrontend extension as it's intended to be used by
the Gather and QuickSurveys extensions.

Bug: T109010
Change-Id: Icf7f6fedf0c2deb5d5548c9e24456cc7a7c6a743