feat(core): add strongly-typed decorators factory (reflector)

[kamilmysliwiec]

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 current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build ac8c3184-b823-4efe-ac62-40fe58c45eca

• 10 of 13 (76.92%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 92.714%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/core/services/reflector.service.ts	10	13	76.92%

Totals
Change from base Build 83148419-bf5c-47e9-b45d-e3f6f40452b6:	-0.03%
Covered Lines:	6401
Relevant Lines:	6904

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💛 - Coveralls

[jmcdo29]

This looks great!