[red-knot] Add basic on-hover to playground and LSP (#17057)
## Summary
Implement a very basic hover in the playground and LSP.
It's basic, because it only shows the type on-hover. Most other LSPs
also show:
* The signature of the symbol beneath the cursor. E.g. `class
Test(a:int, b:int)` (we want something like
https://github.com/microsoft/pyright/blob/54f7da25f9c2b6253803602048b04fe0ccb13430/packages/pyright-internal/src/analyzer/typeEvaluator.ts#L21929-L22129)
* The symbols' documentation
* Do more fancy markdown rendering
I decided to defer these features for now because it requires new
semantic APIs (similar to *goto definition*), and investing in fancy
rendering only makes sense once we have the relevant data.
Closes [#16826](https://github.com/astral-sh/ruff/issues/16826)
## Test Plan
https://github.com/user-attachments/assets/044aeee4-58ad-4d4e-9e26-ac2a712026be
https://github.com/user-attachments/assets/4a1f4004-2982-4cf2-9dfd-cb8b84ff2ecb
