[FEAT] Makes the @action decorator work in classic classes

[pzuraq]

Currently, the new @action decorator is the only decorator in Ember
proper that doesn't work in classic classes currently. Since users could
define actions on the actions hash in classic classes, we didn't
originally see a reason for it to work in classic classes. However, with
the fact that the decorator now binds context, it may actually become
useful for users, especially if they decide to convert away from the
{{action}} modifier/helper.

const FooComponent = Component.extend({
  myAction: action(function() {
    // do a thing
  }),
});

Additionally, this makes them match up with our learning story for the
rest of decorators - they're always decorators, they just use
different syntax in native classes and classic classes.

Also updates setComputedDecorator and isComputedDecorator to setClassicDecorator and isClassicDecorator, since that is a bit more accurate for what they are being used for.