feat: maintain latest ecma version in ESLint

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

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

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

Fixes #15366

What changes did you make? (Give an overview)

I added configuration for the latest ECMAScript version to ESLint and updated Linter to use it instead of espree.latestEcmaVersion when resolving "latest" value of languageOptions.ecmaVersion.

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

I didn't change the eslintrc logic:

eslint/lib/linter/linter.js

Lines 553 to 559 in 9507525

	function normalizeEcmaVersion(parser, ecmaVersion) {
	if (isEspree(parser)) {
	if (ecmaVersion === "latest") {
	return espree.latestEcmaVersion;
	}
	}

because, in eslintrc configs, ecmaVersion is in parserOptions:

// .eslintrc.js
module.exports = {
    parserOptions: {
        ecmaVersion: "latest"
    }
}

thus, in this case, I think it makes more sense to keep getting the version from the parser.

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[snitin315]

LGTM.

I'll leave it open for others to review.

[nzakas]

LGTM. Thanks.