12$: Improve diagnostic for myArray[a:b]

[strager]

For myArray[a:b], we can suggest writing myArray.slice(a, b-a) instead.

Similarly, for myArray[a:], we can suggest writing myArray.slice(a).

Similarly, for myArray[:b], we can suggest writing myArray.slice(0, b).

[Jetoson]

@strager may I work on this?