refactor(core): replace our own 1-level flatten by the native one

[micalevisk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the new behavior?

node12 has Array.prototype.flat method. See https://node.green/#ES2019-features-Array-prototype--flat--flatMap--Array-prototype-flat

thus we don't need to write this logic anymore.

I didn't changed the flatten on @nestjs/common because it is being used by other nestjs packages that didn't bumped their major version (ie., they still supports node10, I guess)

nest/packages/common/decorators/core/dependencies.decorator.ts

Lines 3 to 8 in 34cb600

	export function flatten<T extends Array<unknown> = any>(
	arr: T,
	): T extends Array<infer R> ? R : never {
	const flat = [].concat(...arr);
	return flat.some(Array.isArray) ? flatten(flat) : flat;
	}

the above could be written like this: arr.flat(Infinity)

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Also, I've looked at every JS feature that was added in node12 (taking this table https://node.green) to see if there is something that we could update in this repo. Didn't find other feature tho.

Does this PR introduce a breaking change?

• Yes
• No

[coveralls]

Pull Request Test Coverage Report for Build fe4257cc-19e3-48d4-890d-7603612ac0c3

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.91%

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Totals
Change from base Build 89137624-bda6-49c5-bbc7-d5e6f900f6cc:	0.0%
Covered Lines:	6029
Relevant Lines:	6420

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💛 - Coveralls

[kamilmysliwiec]

lgtm