[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/red_knot_server/src/server.rs

@@ -7,9 +7,9 @@ use std::panic::PanicInfo;
 
 use lsp_server::Message;
 use lsp_types::{
-    ClientCapabilities, DiagnosticOptions, DiagnosticServerCapabilities, MessageType,
-    ServerCapabilities, TextDocumentSyncCapability, TextDocumentSyncKind, TextDocumentSyncOptions,
-    TypeDefinitionProviderCapability, Url,
+    ClientCapabilities, DiagnosticOptions, DiagnosticServerCapabilities, HoverProviderCapability,
+    MessageType, ServerCapabilities, TextDocumentSyncCapability, TextDocumentSyncKind,
+    TextDocumentSyncOptions, TypeDefinitionProviderCapability, Url,
 };
 
 use self::connection::{Connection, ConnectionInitializer};
@@ -221,6 +221,7 @@ impl Server {
                 },
             )),
             type_definition_provider: Some(TypeDefinitionProviderCapability::Simple(true)),
+            hover_provider: Some(HoverProviderCapability::Simple(true)),
             ..Default::default()
         }
     }