Fix for rest parameters in unified-signatures

[sbj42]

PR checklist

• Addresses an existing issue: unified-signatures erroneously claims overloads can be combined #2338
• New feature, bugfix, or enhancement
  • Includes tests

Overview of change:

signaturesDifferByOptionalOrRestParameter() previously considered two signatures similar if the common parameters had the same type, but it did not consider the dotDotDotToken sigil when it dd that comparison. This change has the function check the last parameter of the shorter signature to see if it is a rest parameter. If so, it may not be possible to combine the two signatures.

Is there anything you'd like reviewers to focus on?

This change makes tslint more lenient about a particular scenario:

function rest2(...xs: number[]): void;
function rest2(xs: number[], y: number): void;
function rest2(...args: any[]) {}

tslint previously correctly noted that those signatures could be combined. After this change it does not warn about them. Reviewer should consider whether fixing the false warning of #2338 in exchange for failing to warn in this alternative case is a net improvement.

[bugfix] unified-signatures now recognizes rest parameters

[nchen63]

looks good. thanks