Enforce comparing `typeof` expressions against string literals

I propose adding { "requireStringLiterals": true } to the valid-typeof rule that we already enforce.

This requires typeof expressions to only be compared to string literals, and disallows comparisons to any other value.

Examples of incorrect code:

typeof foo === undefined
typeof bar == Object
typeof baz === 'strnig'
typeof qux === 'some invalid type'
typeof baz === anotherVariable
typeof foo == 5
typeof bar === typeof qux

Examples of correct code:

typeof foo === 'undefined'
typeof bar == 'object'
typeof baz === 'string'

http://eslint.org/docs/rules/valid-typeof

[feross]

This will be part of standard v9. Only one repo was impacted and it was doing this:

const OBJ = 'object'

module.exports = function merge (a, b, checked) {
  if (checked || typeof a === OBJ) {

Can’t see a reason that’s preferable to typeof a === 'object', so not too worried about this case.

[dcousens]

@feross I feel like we don’t thank you enough. Thanks so much for the work, totally understandable. Looking forward to v9.0 😄 👍