[ty] Remove invalid statement-keyword completions in for-statements (#21979)
In `for x in <CURSOR>` statements it's only valid to provide expressions
that eventually evaluate to an iterable. While it's extremely difficult
to know if something can evaulate to an iterable in a general case,
there are some suggestions we know can never lead to an iterable. Most
keywords are such and hence we remove them here.
## Summary
This suppresses statement-keywords from auto-complete suggestions in
`for x in <CURSOR>` statements where we know they can never be valid, as
whatever is typed has to (at some point) evaluate to an iterable.
It handles the core issue from
https://github.com/astral-sh/ty/issues/1774 but there's a lot of related
cases that probably has to be handled piece-wise.
## Test Plan
New tests and verifying in the playground.