Add support for class private methods (#703)

* Add support for class private methods

This commit adds parser support for the TC39 Stage 2 Private Methods
proposal.

This commit also changes "key" in ClassPrivateProperty from an
Identifier to a PrivateName, as well as disallowing #constructor as a
valid private field name.

* Add tests for string literal get/set/async

These should be treated as regular methods and not special get/set/async
behaviour.

* Add tests for class private methods

This also removes a test from the Test262 whitelist that failed before
the changes for private methods support and now passes.

* Modify class private prop tests for PrivateName

* Add class private prop tests for #constructor

* Fix existing ASI test case failure

test/fixtures/experimental/class-private-methods/method/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["classProperties", "classPrivateMethods"]
+}