build: use nyc's merge command
a 'merge' command has recently been added to nyc, eliminating the need for the istanbul-merge library.
both nyc and istanbul-merge rely on istanbul-lib-coverage, using nyc to perform the merge will ensure the same library version is used for instrumenting coverage and merging reports.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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