feat: add improvements in Typescript API

[g4rcez]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Add some utilities for better typing in ObjectSchema.

[aboutlo]

Thank you, @g4rcez, for these improvements!
I will let others have a look, too, but LGTM.

[coveralls]

Pull Request Test Coverage Report for Build 3209062838

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 3177673423:	0.0%
Covered Lines:	403
Relevant Lines:	403

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3209062838

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 3177673423:	0.0%
Covered Lines:	403
Relevant Lines:	403

---

💛 - Coveralls