[framework] Consider moving and renaming `k` constants for improved discoverability

[parlough]

The k constant style (inherited from Flutter) is not standard in the Dart ecosystem and being top-level variables in the already large jaspr.dart library, they're a bit hard to discover.

To improve discoverability and improve consistency of the naming scheme, I'd suggest two things:

1. Moving them to be static fields on Jaspr or a new class such as RunInfo.
2. Changing their names to not have the k prefix, perhaps requiring some more changes as well.

To facilitate this change, the old top-level properties can still remain and be deprecated or provided with an auto migration.

[schultek]

I get your point, but I think the value of having it the same as Flutter still outweighs it.

I like that using 'kIsWeb' and 'kDebugMode' just works.