Update existing rules to handle decorators

[bmish]

For example, computed property rules should handle the @computed decorator.

This would be good preparation for Ember Octane.

Related to #560.

These rules in particular need to be updated:

• require-computed-macros
• require-computed-property-dependencies
• require-return-from-computed

[bmish]

I'm going to close this. At this point, most rules have been updated with support for native JavaScript classes and decorators, including the most important ones. The few remaining rules that haven't been updated note "Missing native JavaScript class support" in their rule docs.