Update: support "bigint" in valid-typeof rule

[cjihrig]

BigInt just moved to Stage 4, so add support for it to the valid-typeof rule.

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[x] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What rule do you want to change?
valid-typeof

Does this change cause the rule to produce more or fewer warnings?
fewer

How will the change be implemented? (New option, new default behavior, etc.)?
A new valid typeof value, 'bigint', is added.

Please provide some example code that this change will affect:

if (typeof foo === 'bigint') {
  console.log('bigint');
}

What does the rule currently do for this code?
Reports an error.

What will the rule do after it's changed?
Accept bigint as a valid typeof value.

What changes did you make? (Give an overview)
Added a new typeof value to the rule, test, and docs.

Is there anything you'd like reviewers to focus on?
Nothing particular.

[aladdin-add]

LGTM, thanks!

[platinumazure]

LGTM, thanks!