New: require-await rule (fixes #6820)

[mysticatea]

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

[X] New rule (template)

Please describe what the rule should do:

This rule disallows async functions which have no await expression.
Async functions which have no await expression may be the unintentional result of refactoring.

See #6820 also.

What category of rule is this? (place an "X" next to just one item)

[X] Enforces code style
[ ] Warns about a potential error
[ ] Suggests an alternate way of doing something
[ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about:

/*eslint require-await: error*/

//------------------------------------------------------------------------------
// ✔ GOOD
async function foo() {
    await doSomething()
}

// This rule ignores empty functions as same as `require-yield` rule.
async function noop() {
    // do nothing.
}

function foo() {
    doSomething()
}

//------------------------------------------------------------------------------
// ✘ BAD
async function foo() {
    doSomething()
}

Why should this rule be included in ESLint (instead of a plugin)?

• This is a general matter for JavaScript language.
• ESLint has require-yield rule which does the same check for generator functions.

What changes did you make? (Give an overview)

This PR adds the require-await rule into core.

In addition, this PR adds 2 functions to ast-utils.

• getFunctionNameWithKind(node) gets the name and kind of the given function. I intended that this is an utility to use for Include function name in report message if possible / relevant #7260. Please see JSDoc comment for details.
• getFunctionHeadLoc(node, sourceCode) gets the location of the function head. I intended that this is an utility to use for Return range in the lint result #3307. Please see JSDoc comment for details.

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

• Messages and documents.
• Added 2 utilities.

[mention-bot]

@mysticatea, thanks for your PR! By analyzing the history of the files in this pull request, we identified @nzakas, @kaicataldo and @gyandeeps to be potential reviewers.

[eslintbot]

LGTM

[platinumazure]

One @fileoverview needs to be fleshed out, and I identified another possible ast-utils candidate, otherwise looks good.

[platinumazure]

This needs to be fleshed out, I think. 😄

[mysticatea]

Oops, I have forgotten it since I use this template after a long time.

[platinumazure]

Seems like this should go in ast-utils?

[mysticatea]

Probably.

[eslintbot]

LGTM

[nzakas]

Just a heads up: we have moved to a new CLA checker on pull requests. Even if you've previously signed our CLA, we will need to you sign the new one. To do so, look at the status checks for licence/cla and click the "Details" link. Sorry for the inconvenience.

[vitorbal]

@platinumazure I believe your concerns have been addressed in the new commits from @mysticatea . Could you take a look when you have a chance?