[webdev] Generate auto-switching combined build for dart2js and dart2wasm output

I'm not exactly sure at what layer this should be in, but `webdev` seems like an most appropriate place. Happy to hear what other's think! Perhaps it does make sense directly as a `dart` subcommand, not sure.

Building on https://github.com/dart-lang/sdk/commit/0df58fc2d07107846ec5fb28764283b32e6bd1dc and eventually https://github.com/dart-lang/build/issues/3621:

I think `webdev` should have a release mode that generates output with both `dart2js` (`dart compile js`) and `dart2wasm` (`dart compile wasm`). Then it should generate a JS script which chooses between them based on browser capabilities.

Developers can technically do this themselves, but I imagine it will be common enough that it's worth having some built-in functionality to do so.

Perhaps for a while, this can be a separate mode from the existing `--release`, but eventually I'd expect this to be the default.

\cc @kevmoo @jakemac53 For general thoughts, including about where this functionality could live.