Update phase of the custom-page-sizes proposal in `features.json`

[fitzgen]

The proposal advanced to phase 2 in today's CG meeting.

@tlively PTAL, thanks!

[dschuff]

LGTM, but it's possible that #390 will land soon, which would also cover this. so maybe let's hold off for a couple of days and maybe we won't need this and we can avoid causing merge conflicts.

[fitzgen]

Ah cool! I hadn't seen that PR.

Would you mind rubber stamping https://github.com/WebAssembly/proposals/pull/194 in the meantime?

[chicoxyzzy]

#390 has been merged, so I think we can close this one