Grammar, Nodes: Class fields/class property initializers

We should be able to write the equivalent of
```js
class C {
b = 3;
c = () => 4;
}
```
Can it be
```coffee
class C
b: 3
c: () => 4;
```
or does the `b` property have different semantics?

Start with the grammar and Nodes areas implicated by the title, and compare the requested CoffeeScript examples with their JavaScript class-field equivalents. Resolve whether instance property initializers have the same semantics, then define the accepted syntax and its expected output; the issue provides no file or test path.

Written by the indexing model from the issue text.