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_multiple.carbon

@@ -0,0 +1,33 @@
+// 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: result: 0
+
+package ExplorerTest api;
+
+base class A {
+  fn FunctionA() {}
+  var var_a: i32;
+}
+
+base class B extends A {
+  fn FunctionB() {}
+  var var_b: i32;
+}
+
+class C extends B {
+  fn FunctionC() {}
+  var var_c: i32;
+}
+
+fn Main() -> i32 {
+  var c: C = {.var_a=1, .var_b=2, .var_c=3};
+  c.FunctionA();
+  c.FunctionB();
+  c.FunctionC();
+  return 0;
+}