build: build cordova.js during npm prepare

[raphinesse]

Motivation and Context

This will build cordova.js from source using npm's prepare hook.

Description

• build cordova.js from source using npm's prepare hook
• copy prepared cordova.js before running Java tests
• delete and ignore the two prebuilt cordova.js files we had
• do not bundle cordova-js-src anymore, since support for browserify builds has been discontinued for a while now

Testing

• successfully built tarball using npm pack
• successfully ran npm cit from clean repo
• the generated cordova.js is identical to the previously bundled one (sans a few comments)

TODO

• use cordova-js@6.1 once it is published

[codecov-commenter]

Codecov Report

Merging #1271 (bafb62a) into master (adc380c) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1271      +/-   ##
==========================================
- Coverage   73.26%   73.25%   -0.02%     
==========================================
  Files          21       21              
  Lines        1646     1645       -1     
==========================================
- Hits         1206     1205       -1     
  Misses        440      440              

Impacted Files	Coverage Δ
lib/create.js	94.44% <ø> (-0.04%)	⬇️

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