Fix: throw on root-level required

[erfanium]

Fixes #132
I added isRoot option for valueOf method, which is true by default and false for internal calls.
I didn't update .d.ts files, should I do?

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

[coveralls]

Pull Request Test Coverage Report for Build 900830188

• 0 of 5 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 99.559%

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Totals
Change from base Build 892669691:	0.005%
Covered Lines:	410
Relevant Lines:	410

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 903541000

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 7 of 7 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 99.559%

---

Totals
Change from base Build 892669691:	0.005%
Covered Lines:	410
Relevant Lines:	410

---

💛 - Coveralls

[aboutlo]

tks @erfanium! LGTM

[mcollina]

lgtm