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_variables.carbon

@@ -0,0 +1,39 @@
+// 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: Read var_c=1
+// CHECK:STDOUT: Read var_d=2
+// CHECK:STDOUT: Assign var_c=3
+// CHECK:STDOUT: Assign var_d=4
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+base class C {
+  var var_c: i32;
+}
+
+class D extends C {
+  var var_d: i32;
+}
+
+fn Main() -> i32 {
+  // Initialization
+  var d: D = {.var_c= 1, .var_d= 2};
+
+  // Read
+  Print("Read var_c={0}", d.var_c);
+  Print("Read var_d={0}", d.var_d);
+
+  // Assignment
+  d.var_c = 3;
+  d.var_d = 4;
+  Print("Assign var_c={0}", d.var_c);
+  Print("Assign var_d={0}", d.var_d);
+
+  return 0;
+}