This is one part of a WIP implementation of the stage-2 proposal to add
fields to classes: https://github.com/tc39/proposal-class-public-fields

See design doc:
https://docs.google.com/document/d/1WRtNm3ZLNJT1WVr8aq4RJuByYgfuAFAhj20LwTW6JVE/

This adds support for parsing fields in classes, including
infrastructure. In particular, it adds:
* Two booleans on function literal AST nodes
* Two compiler hints on SharedFunctionInfos representing said bools
* A new type of ClassLiteralProperty, FIELD
* Parser support for the syntax
* Syntax tests
* A flag to enable it.

Currently the fields are parsed and then droppped. Subsequent
patches will add semantics, mostly by desugaring in the parser and
the remainder in the non-crankshaft backends.

BUG=v8:5367

Review-Url: https://codereview.chromium.org/2315733003
Cr-Commit-Position: refs/heads/master@{#39459}