Replace underscore with modern JS

[raphinesse]

Because we can. The only logical difference should be that Object.assign takes own enumerable props, while _.extend enumerable.

Resolves #643

[codecov-io]

Codecov Report

Merging #772 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
- Coverage   85.71%   85.63%   -0.09%     
==========================================
  Files          50       50              
  Lines        2598     2582      -16     
==========================================
- Hits         2227     2211      -16     
  Misses        371      371

Impacted Files	Coverage Δ
src/cordova/compile.js	100% <ø> (ø)	⬆️
src/plugman/variable-merge.js	100% <100%> (ø)	⬆️
src/cordova/platform/addHelper.js	95.54% <100%> (-0.03%)	⬇️
src/cordova/prepare.js	100% <100%> (ø)	⬆️
src/cordova/run.js	100% <100%> (ø)	⬆️
src/cordova/emulate.js	100% <100%> (ø)	⬆️
src/plugman/util/dependencies.js	92.1% <100%> (-1.08%)	⬇️
src/plugman/uninstall.js	93.5% <100%> (-0.05%)	⬇️
src/plugman/install.js	90.9% <100%> (-0.04%)	⬇️
src/plugman/fetch.js	86.06% <100%> (-0.12%)	⬇️

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[dpogue]

👍 from me

[erisu]

👍

[erisu]

Nice! 👍

As part of the work I been doing on the draft branched for all tooling repos, I been looking to deprecate the underscore dependency.

I forgot which repo exactly I was looking at, at the time, but good to see and know that this PR already existed.

When I started the draft branches I forgot to check all PRs :D.