Fix handling of class property value this in module transforms. (#9857)

Prevent module transforms from rewriting `this` to `void 0` when found in the values of private class properties.  Add tests to prevent regressions.

packages/babel-helper-module-transforms/src/rewrite-this.js

@@ -17,4 +17,7 @@ const rewriteThisVisitor = {
   ClassProperty(path) {
     path.skip();
   },
+  ClassPrivateProperty(path) {
+    path.skip();
+  },
 };