Add new impliedByAfterDate annotation to compatibility-date

[jasnell]

The new annotation allows us to specify that a compatibility flag is implied if the named other flag is enabled, but only at the specified compatibility date or later. If the flag is marked experimental and experimental flags are not allowed, or if the flag is explicitly turned off, the annotation will not apply.

This will be used, for instance, to specify that nodejs_compat_v2 is implied on by nodejs_compat after a given date.

[irvinebroque]

To confirm, what this lets us then provide is:

• If you are using nodejs_compat today, and have a compat date of 06-25-24 — nothing changes
• If you are then change your compat date to <future date> — we will (behind the scenes) use nodejs_compat_v2 instead

...allowing us to maintain existing behavior, and provide an explicit opt-in (via a compatibility date)

[jasnell]

Yes, that's exactly it @irvinebroque

[kentonv]

Code looks good but can we add a test to compatibility-date-test.c++? Can base the test around the nodejs_compat_v2 flag specifically.

[jasnell]

Test added. experimental check removed.

[IgorMinar]

I can't review the c++ code, but the tests look exactly like what I would expect. I just have some minor product comments.