Implement AstNode for Identifier (#13207)

## Summary

Follow-up to #13147, this PR implements the `AstNode` for `Identifier`.
This makes it easier to create the `NodeKey` in red knot because it uses
a generic method to construct the key from `AnyNodeRef` and is important
for definitions that are created only on identifiers instead of
`ExprName`.

## Test Plan

`cargo test` and `cargo clippy`

crates/ruff_python_formatter/src/range.rs

@@ -703,6 +703,7 @@ impl Format<PyFormatContext<'_>> for FormatEnclosingNode<'_> {
             | AnyNodeRef::TypeParamTypeVar(_)
             | AnyNodeRef::TypeParamTypeVarTuple(_)
             | AnyNodeRef::TypeParamParamSpec(_)
+            | AnyNodeRef::Identifier(_)
             | AnyNodeRef::BytesLiteral(_) => {
                 panic!("Range formatting only supports formatting logical lines")
             }