chore: replace nyc with c8 for code coverage

[JKRhb]

Having a look at coveralls.io, I noticed that code coverage seems to be a bit broken right now. I recently found a better tool than nyc called c8 that seems to work with V8's native code coverage capabilities and be used as a drop-in replacement, fixing typescript related issues with nyc. It also gets rid of one additional dev-dependency.

[coveralls]

Pull Request Test Coverage Report for Build 2212862587

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 97 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+7.3%) to 94.556%

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Files with Coverage Reduction	New Missed Lines	%
lib/outgoing_message.ts	1	97.92%
index.ts	2	93.43%
lib/helpers.ts	2	97.07%
lib/observe_read_stream.ts	2	92.21%
lib/observe_write_stream.ts	2	95.1%
lib/cache.ts	3	92.0%
lib/segmentation.ts	3	89.58%
lib/option_converter.ts	4	94.39%
lib/middlewares.ts	5	87.4%
lib/agent.ts	28	87.13%

Totals
Change from base Build 2201295132:	7.3%
Covered Lines:	8660
Relevant Lines:	9060

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💛 - Coveralls

[JKRhb]

Oh, as you can see the improvement above already :) nyc wasn't capable of handling typescript, as it seems.