Return correct frame from mw.getCurrentFrame in certain edge cases
When an #invoke is passed as an argument to another #invoke,
mw.getCurrentFrame() at module scope will return the wrong frame.
On the PHP side, we need to always reset the frame when processing
an #invoke, not just when there's no frame already. I don't remember why
I82dde43e wasn't done that way, but changing it doesn't make any tests
fail and Scribunto tends to have good tests.
On the Lua side, we need to do the same. The logic wih mw.getCurrentFrame()
using a global that gets stored, modified, and reset in several places
was getting confusing, so this patch reworks the logic to inject a
globalless mw.getCurrentFrame() into each #invoke's cloned environment