Coexist with Decorators

[iccir]

Similar to "use strict", we should have a directive that enables the use of the NilScript feature superset (and would be a superset of Strict Mode).

Certain ECMAScript features could be disabled in "NilScript Mode", for example: decorators. This allows NilScript to coexist with ECMAScript when they define syntax that we already use.

[iccir]

This would transpile to "use strict" for scripts and be removed for modules.

[iccir]

This is probably premature at this point:

1. Decorators need to get to Stage 3 (and then 4).
2. Esprima needs to add decorator support.