Stage 3 decorators are currently standard in TypeScript and Babel. We're

not able to use it in Spina just yet, but in preparation, we're now

adding the context argument to our decorator function to signal

support for Stage 3 decorators. It's made optional, allowing legacy

decorators to continue to work.
The decorators are also typed with <This, TBase>, which allows us to

return a proper type mixing in the decorated class.
There are also some documentation fixes around decorator arguments.

Unit tests pass with legacy decorators.