Negation syntax

[leoj3n]

I agree with this comment:

systemjs/systemjs#285 (comment)

However, I think a better negation syntax would be ^ instead of ~, especially since steal supports ~/ for referencing modules relative to the project root (which makes the current ~ negation syntax confusing) and ^ would have greater parity with stache:

https://canjs.com/doc/can-stache.tags.inverse.html