[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
54f7da25f9/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

crates/ruff_python_parser/src/lib.rs

@@ -690,6 +690,7 @@ impl Deref for Tokens {
 }
 
 /// A token that encloses a given offset or ends exactly at it.
+#[derive(Debug, Clone)]
 pub enum TokenAt {
     /// There's no token at the given offset
     None,