[ty] Add return-type inlay hints (#29437)
## Summary
Show return-type inlay hints for unannotated functions in the playground
and in editors that support it:
<img width="478" height="278" alt="image"
src="https://github.com/user-attachments/assets/52145f44-ab5d-4fc1-8621-d54207d49454"
/>
Return types can also be "baked in". After double clicking:
<img width="478" height="278" alt="image"
src="https://github.com/user-attachments/assets/3d6ee13c-6cd7-4a1a-b1f2-e347030ddffe"
/>
And here it is in VS Code (baking in works as well):
<img width="580" height="148" alt="image"
src="https://github.com/user-attachments/assets/c59725fe-22e4-4eac-b534-6bb9561efdec"
/>
## Test plan
New snapshot tests. Also tested interactively in the playground and in
VS Code.
