Add mw.isSubsting()

It's already possible to detect whether the current template is being
substituted via ParserFunctions (see [[en:Template:Ifsubst]]), and a
similar trick works with frame:preprocess. So we may as well provide the
flag directly.

Bug: 47828
Change-Id: Id06d27c6283ee589a8830b78c04e56978e0ac6da
Merge "Add mw.isSubsting()"