Support `@virtual` annotation on fields to allow field overrides in strong mode

[leafpetersen]

The current long term plan is to allow field overrides in strong mode with an opt out syntax for sealed fields. In the meantime, strong mode should support an opt-in for field overrides via an @virtual annotation.

@sethladd @Hixie @abarth

[bwilkerson]

Is there a reason why strong mode can't allow field overrides in the short term, rather than to introduce support that will require a breaking change to remove?

[leafpetersen]

We plan to have actual syntax, so no matter which annotation which choose now (@sealed, @virtual) removing it will be a breaking change. In the meantime, since DDC is the only platform for which these have semantic meaning, I'd like to keep sealed as the default since this will give our users a better experience on DDC.

[jmesserly]

@leafpetersen also before we change the default, we should automated refactor our internal code base to seal-all-the-things. (edit: in other words, those fields are already safe to seal, so we might as well get the DDC size+perf benefit from that, rather than regress it.)

[jmesserly]

note: also need to check that @checked interacts correctly with this (#27363)

[jmesserly]

https://codereview.chromium.org/2352433002/