[ty] Stabilize auto-import
While still under development, it's far enough along now that we think
it's worth enabling it by default. This should also help give us
feedback for how it behaves.
This PR adds a "completion settings" grouping similar to inlay hints. We
only have an auto-import setting there now, but I expect we'll add more
options to configure completion behavior in the future.
Closes astral-sh/ty#1765
