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/class_inheritance_methods.carbon

@@ -0,0 +1,38 @@
+// 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: %{explorer-run}
+// RUN: %{explorer-run-trace}
+// CHECK:STDOUT: Class 1
+// CHECK:STDOUT: Class 2
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+base class C {
+  fn BasePrint(v: i32) {
+    Print("Class {0}", v);
+  }
+  fn Method1[me:Self]() {
+    me.BasePrint(me.value_c);
+  }
+
+  var value_c: i32;
+}
+
+class D extends C {
+  fn Method2[me:Self]() {
+    me.BasePrint(me.value_d);
+  }
+
+  var value_d: i32;
+}
+
+fn Main() -> i32 {
+  var d: D = {.value_c = 1, .value_d = 2};
+  d.Method1();
+  d.Method2();
+  return 0;
+}