Update to use nyc@15 instead of istanbul.

[the-t-in-rtf]

We currently collect code coverage with istanbul. Running with nyc results in errors like:

Error: Assertion failure - check console for more details: Assertion failure - check console for more details: Assertion failure - check console for more details: Assertion failure - check console for more details: javascript error: cov_98otombds is not defined
JavaScript stack:
ReferenceError: cov_98otombds is not defined

As part of updating to support selenium-webdriver@4 I would suggest refactoring the coverage setup to use nyc and working through the issues.