This patch implements https://github.com/tc39/proposal-class-fields/pull/65
and https://github.com/tc39/proposal-static-class-features/ by
splitting out instance and static field declarations into separate
flags for the separate proposals. Instance class fields is currently
at Stage 3 whereas static class fields is currently at Stage 2.

Bug: v8:5367
Change-Id: I133c945fd0b22dc5718c7bb61b10f22348087acd
Reviewed-on: https://chromium-review.googlesource.com/839778
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50293}