resourceloader: Add getScript() method for loading scripts with callback

resourceloader: Add getScript() method for loading scripts with callback

Provides a friendly wrapper for loading scripts using $.ajax().
Returns a promise when the script dependency -- a single
script url -- has been loaded. This matches how mw.loader.load
is able to load a script url, and how mw.loader.using will
return a promised that is resolved when dependecies are loaded,
or rejected if there is an error.

Added as a separate function mw.loader.getScript(), rather
than adding this functionality to using(), as it is a separate
use case, less confusing for users, and there is no shared or
duplicated code between loading a script and loading one
or more named modules.

Bug: T27962
Change-Id: I13be426d03261a2d0c6a1631af94a9f9af58394b