Adds a catch-all experimental compatibility flag for incremental work

[jasnell]

Will be used, for instance, to incrementally work on experimental Node.js compatibility stuff separately from the nodejs_compat flag that will be used to enable the stuff that's stable.

[jasnell]

Not 100% convinced this is the right direction. @kentonv had suggested the catch-all as a possible approach so wanted to write it up really quick. Do we really want to do it this way or have more specific flags?

[kentonv]

Yes, this is what I had in mind.

To explain, the idea here is we'd use this in cases where the change being guarded is expected to be backwards-compatible, and thus no compat flag is needed in the long term, but we still want to block use of the API until it stabilizes. This way we don't end up with a proliferation of obsolete compat flags.

I still like this idea but am open to arguments why we wouldn't want to do it this way...

[jasnell]

open to arguments why we wouldn't want to do it this way...

Just a general concern about it potentially being abused, but no strong feelings either way.

[jasnell]

Rebased to clear the merge conflict

[jasnell]

Let's not land this yet until the discussion about the nodejs-compat flag is settled. We may be able to repurpose that flag instead of adding a new one. Ok, looks like we're going to stick with the nodejs-compat flag

[jasnell]

recheck