This patch implements the parsing of private methods
in the stage 3 proposal https://tc39.github.io/proposal-private-methods

- Adds a --harmony-private-methods flag
- Parse the private methods/accessors

The design doc is in
https://docs.google.com/document/d/1T-Ql6HOIH2U_8YjWkwK2rTfywwb7b3Qe8d3jkz72KwA/edit?usp=sharing

This patch only makes sure the syntax parses, doesn't implement
the semantics.

Bug: v8:8330
Change-Id: I9007b3b3dd6a0df35db7bb14f38f1a38d52bc663
Reviewed-on: https://chromium-review.googlesource.com/c/1329706
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57615}