build: use c8 and drop lock file

[JustinBeckwith]

No description provided.

[codecov]

Codecov Report

Merging #135 into master will increase coverage by 11.37%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #135       +/-   ##
===========================================
+ Coverage   80.86%   92.24%   +11.37%     
===========================================
  Files          13        5        -8     
  Lines         162      580      +418     
  Branches       34      119       +85     
===========================================
+ Hits          131      535      +404     
- Misses         28       43       +15     
+ Partials        3        2        -1

Impacted Files	Coverage Δ
src/gaxios.ts	87.71% <0%> (-9.63%)	⬇️
src/retry.ts	90.47% <0%> (-9.53%)	⬇️
src/index.ts	97.36% <0%> (-2.64%)	⬇️
src/common.ts	100% <0%> (ø)	⬆️
system-test/fixtures/sample/webpack.config.js
webpack.config.js
webpack-tests.config.js
prettier.config.js
.jsdoc.js
samples/quickstart.js
... and 3 more

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