Implement 4201: add option to ignore shadowed underscore

[rrogowski]

PR checklist

• Addresses an existing issue: fixes no-shadowed-variable should ignore shadowed _-params #4201
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Add option to ignore shadowed underscore.

Usage

{
  "rules": {
    "no-shadowed-variable": [true, {"underscore": false}]
  }
}

Before

import _ = require('lodash');
function foo(_) {
             ~ [Shadowed name: '_']
    console.log(_);
}

After

import _ = require('lodash');
function foo(_) {
    // No lint error
    console.log(_);
}

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

Have I made all the necessary changes to documentation?

CHANGELOG.md entry:

[enhancement]: added option to no-shadowed-variable to ignore shadowed underscores

[JoshuaKGoldberg]

Thanks for starting this! I think we should expand the allowed nameset a bit.

[rrogowski]

@JoshuaKGoldberg suggestions have been addressed. Ready for your review.

[JoshuaKGoldberg]

Awesome work, thanks!