[red-knot] Understand typing.Callable (#16493)

## Summary

Part of https://github.com/astral-sh/ruff/issues/15382

This PR implements a general callable type that wraps around a
`Signature` and it uses that new type to represent `typing.Callable`.

It also implements `Display` support for `Callable`. The format is as:
```
([<arg name>][: <arg type>][ = <default type>], ...) -> <return type>
```

The `/` and `*` separators are added at the correct boundary for
positional-only and keyword-only parameters. Now, as `typing.Callable`
only has positional-only parameters, the rendered signature would be:

```py
Callable[[int, str], None]
# (int, str, /) -> None
```

The `/` separator represents that all the arguments are positional-only.

The relationship methods that check assignability, subtype relationship,
etc. are not yet implemented and will be done so as a follow-up.

## Test Plan

Add test cases for display support for `Signature` and various mdtest
for `typing.Callable`.

crates/ruff_python_ast/src/nodes.rs

@@ -2179,6 +2179,13 @@ impl ExprName {
     pub fn id(&self) -> &Name {
         &self.id
     }
+
+    /// Returns `true` if this node represents an invalid name i.e., the `ctx` is [`Invalid`].
+    ///
+    /// [`Invalid`]: ExprContext::Invalid
+    pub const fn is_invalid(&self) -> bool {
+        matches!(self.ctx, ExprContext::Invalid)
+    }
 }
 
 impl ExprList {