strict-boolean-expressions: new option allow-boolean-or-undefined

[ajafff]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

I find it weird to compare for strict equality on unions of boolean and undefined. Consider the following function:

function foo(bar?: boolean) {
    if (bar) // not allowed by default
       // do stuff
}

allow-undefined-union would allow too much, because it also allows for example ts.Node | undefined.

It's already awkward to use ts.NodeArray#hasTrailingComma === true. With my recent changes to typescripts typings (microsoft/TypeScript#15887, microsoft/TypeScript#15903), that will hit us with the next version of typescript, this pattern will occur in many more places (e.g. ts.forEachChild).
That's why I added this new option allow-boolean-or-undefined.

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

CHANGELOG.md entry:

[new-rule-options] strict-boolean-expressions adds allow-boolean-or-undefined