`dart2wasm`: Export constant identifying the compiler about sdk

[lrhn]

As you link to #54785, this is something that can be useful for more than just Dart2Wasm.
I'd prefer a general solution to something that only works for one compiler.
(Preferably entries that distinguish, at least, whether running natively, in JS or in Wasm, and possibly also whether compiled in development mode or production mode, and whether asserts are enabled.)

That is, I'd want to also define dart.tool.vm, dart.tool.dart2js, dart.tool.dev_compiler (or whatever names they want to be known under).
I believe dart.tool.dart2js already exists. The rest should too.

(But I'm OK with just agreeing that they should all exist, and then landing this as the first step.)

[JaffaKetchup]

Need this here as well! Could be very useful for example projects for packages to show an indicator as to whether it's running with WASM, to verify potential performance improvements.