Give a better error when using a 'this' parameter in an arrow function.

[DanielRosenwasser]

let f = (this: number) => 100;

Currently this gives an error on the : saying that ) was expected. Instead, we should

1. Gracefully parse.
2. Give a grammar error saying that this parameters are not allowed in arrow functions.

Source: a user ran into this hazard here.

[tony19]

Also note the change in error when noImplicitThis is enabled.

• Without noImplicitThis, the error is:

  ')' expected
  

• With noImplicitThis, the error is:

  'this' implicitly has type 'any' because it does not have a type annotation
  

[DanielRosenwasser]

Looks like a duplicate of #9744.

Thanks @mattmccutchen!