feat: upgrade to version of yargs with extend support

[bcoe]

Along with upgrading a few other dependencies, I've finally upgraded us to the version of yargs with @rcoy-v's extends support.

@rcoy-v mind giving this a try; you can test it out by running:

npm i nyc@next --save-dev

[rcoy-v]

I've tested the following scenarios:

1. .nycrc extending another .nycrc from node_modules
   Works.

2. package.json "nyc" extending a .nycrc from node_modules
   Does not work.

If you could make a checklist or something outlining all the scenarios you expect should work, I will test them.

[bcoe]

@rcoy-v huh; I could have sworn we'd tested that scenario in yargs;

I'll land this feature, since we've been waiting quite some time, and it's probably worth us opening an issue on yargs.

[rcoy-v]

@bcoe, I opened this issue on yargs with what I believe is the cause.