Use dedicated AST nodes on MemberKind (#6374)

## Summary

This PR leverages the unified function definition node to add precise
AST node types to `MemberKind`, which is used to power our docstring
definition tracking (e.g., classes and functions, whether they're
methods or functions or nested functions and so on, whether they have a
docstring, etc.). It was painful to do this in the past because the
function variants needed to support a union anyway, but storing precise
nodes removes like a dozen panics.

No behavior changes -- purely a refactor.

## Test Plan

`cargo test`

crates/ruff_python_ast/src/identifier.rs

@@ -20,6 +20,32 @@ pub trait Identifier {
     fn identifier(&self) -> TextRange;
 }
 
+impl Identifier for ast::StmtFunctionDef {
+    /// Return the [`TextRange`] of the identifier in the given function definition.
+    ///
+    /// For example, return the range of `f` in:
+    /// ```python
+    /// def f():
+    ///     ...
+    /// ```
+    fn identifier(&self) -> TextRange {
+        self.name.range()
+    }
+}
+
+impl Identifier for ast::StmtClassDef {
+    /// Return the [`TextRange`] of the identifier in the given class definition.
+    ///
+    /// For example, return the range of `C` in:
+    /// ```python
+    /// class C():
+    ///     ...
+    /// ```
+    fn identifier(&self) -> TextRange {
+        self.name.range()
+    }
+}
+
 impl Identifier for Stmt {
     /// Return the [`TextRange`] of the identifier in the given statement.
     ///
@@ -30,8 +56,8 @@ impl Identifier for Stmt {
     /// ```
     fn identifier(&self) -> TextRange {
         match self {
-            Stmt::ClassDef(ast::StmtClassDef { name, .. })
-            | Stmt::FunctionDef(ast::StmtFunctionDef { name, .. }) => name.range(),
+            Stmt::ClassDef(class) => class.identifier(),
+            Stmt::FunctionDef(function) => function.identifier(),
             _ => self.range(),
         }
     }