It's not a keyword outside of generator functions, so extend the
qmlmode boolean on the lexer to become a more general parsemode that
we can use to also turn the yield keyword on and of.
The parser can then set the flag when it enters the body of a generator
function.
Change-Id: Ibf792d4c7c567d825c6706f7b4997362c87fc575
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
4e18d38f16