Add support for shader builtins and enable the extension.

Bug: angleproject:3578
Tests: dEQP-GLES31.functional.*image_atomic*
Change-Id: Idd45b2ee62efe1474c6c5947c77da64ff2221bf6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2505540
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>