src: update RegExp type constant for V8 8.1

[mmarchini]

This is the last change needed to support Node.js v14.

[mmarchini]

Landed in dd57bfb8af3c...4918962beed7

[coveralls]

Pull Request Test Coverage Report for Build 84284346

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report. To ensure accuracy in future PRs, please see these guidelines. A quick fix for this PR: rebase it; your next report should be accurate.

• 4 of 4 (100.0%) changed or added relevant lines in 3 files are covered.
• 213 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+3.8%) to 78.77%

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Files with Coverage Reduction	New Missed Lines	%
src/llv8-constants.cc	2	85.43%
src/llv8.cc	3	71.29%
test/common.js	7	80.08%
src/llnode_api.cc	8	87.64%
src/node-inl.h	11	0.0%
src/node.h	12	20.0%
test/plugin/workqueue-test.js	13	55.17%
src/node.cc	21	39.02%
src/node-constants.cc	38	32.86%
src/llnode.cc	42	59.62%
<!--	Total:	213		-->

Totals
Change from base Build 84248866:	3.8%
Covered Lines:	3747
Relevant Lines:	4744

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💛 - Coveralls