[feature] strict Promise configuration validation

[jbottigliero]

• updates configuration schema to use "instanceof": "Promise" (configuration validation : strict "Promise" instanceof validation #258)

What kind of change does this PR introduce?
feature/refactoring

Did you add tests for your changes?
Covered by existing tests.

If relevant, did you update the documentation?
n/a

Summary
Previously, validation for Promise configurations passed via "instanceof": "Function", with webpack@4+ (ajv-keywords@3+), we can do strict validation on "instanceof": "Promise".

To accommodate this change we have to change the check to anyOf due to Promise passing "type": "object".

> const Ajv = require('ajv');
undefined
> const ajv = new Ajv;
undefined
> ajv.validate({type:'object'}, new Promise(res => res));
true

[jbottigliero]

Didn't realize commitlintwas enabled now... amended my commit message to follow the spec.

[dhruvdutt]

@jbottigliero You'll need to use one of these to pass commit lint and CI build.

Eg. misc(config): strict Promise configuration validation

webpack-cli/.cz-config.js

Lines 20 to 29 in 191b458

	{value: 'new_feature', name: 'new_feature: A new feature'}, {
	value: 'ast',
	name: 'ast: init, migrate, add, etc'
	},
	{value: 'tests', name: 'tests: Tests, jest, binTestCases etc'},
	{value: 'docs', name: 'docs: Documentation'},
	{value: 'deps', name: 'deps: Dependency bumps only'},
	{value: 'cli', name: 'cli: core CLI things'},
	{value: 'misc', name: 'misc: Other formats like tweaks and such'},
	{value: 'chore', name: 'chore: Updating docs, linting etc'},

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[EugeneHlushko]

totally legit 👍