Explorer: Support class extension (#1946)

Support class extension with `extends`
Relates to #1881 

Add support for:
* Class extension with `extends`
* Using base class methods and members

explorer/testdata/class/fail_extends_final_class.carbon

@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package ExplorerTest api;
+
+class C {
+  fn F() {}
+}
+
+class D extends C {
+  fn G() {}
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_extends_final_class.carbon:[[@LINE+1]]: Base class `C` is `final` and cannot inherited. Add the `base` or `abstract` class prefix to `C` to allow it to be inherited
+}
+
+fn Main() -> i32 {
+  return 0;
+}