Fix #4464: backticked expressions in class body

[GeoffreyBooth]

Fixes #4464, and allows backticks to provide a workaround for class properties (#4552).

This PR treats backticked expressions in a class body as a special case that doesn’t get hoisted; they just get output in the class body:

class A
  `b: 3`
  c: -> 4

var A;

A = class A {
  b: 3;

  c() {
    return 4;
  }

};

This allows backticks to be used for the not-yet-finalized feature of class properties, as shown above (#4552); or any other unsupported code people might want to put in a class body:

class A
  `get x() { return 42; }`

var A;

A = class A {
  get x() { return 42; };
};

The special case is triggered only when the entire expression is backticked. A line that mixes backticked and unbackticked code, assuming it doesn’t throw a compiler error, will be output as it is now (in the hoisted class body). And the backticked expressions are always output first, above the rest of the class body. @connec @xixixao

[connec]

@GeoffreyBooth as I mentioned to you I found a much smaller change that accomplishes the same thing.

Edit: I rolled back most of the other changes since they seem to have been 'side-effects' of that approach.

[GeoffreyBooth]

A simpler solution would be great! That's why I wanted you to look at this 😄

Can you push your version on this branch, to update the PR?

[connec]

@GeoffreyBooth I did already 😄 https://github.com/jashkenas/coffeescript/pull/4668/files

[GeoffreyBooth]

I don't see your commits.

[connec]

I had squashed the original one, but I've split it out now.

[GeoffreyBooth]

Is this comment no longer valid?

[connec]

I wavered on it a little. I felt it was confusing since the check now also allows PassthroughLiterals, and I didn't want to add that to the comment.

[connec]

I guess I could go the other way and flesh it out a little to indicate that it's the key method for determining whether something appears in the class initializer or executable body?

[GeoffreyBooth]

Yeah I think more is better. Maybe even explicitly mention class fields/class properties, that when they reach Stage 4 this is where we detect them. That way future PRs to add things like that look more like your version of this PR and less like mine 😄

[connec]

It seems I broke the PR by pushing to origin/master by mistake 😢

I reverted origin/master and reopened #4712