dep: replace underscore w/ lodash module pkgs

[erisu]

Motivation and Context

Drop the entire underscore package.

Description

Only a few methods from the package was being used. To reduce the size footprint, I replaced with various lodash modules.

• _.extend -> lodash.assign
• _.isDate -> lodash.isdate
• _.isObject -> lodash.isobject
• _.zip -> lodash.zip

Testing

• npm t

Checklist

• [x] I've run the tests to see all new and existing tests pass
• [ ] I added automated test coverage as appropriate for this change
• [ ] Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• [ ] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [ ] I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Merging #193 (cfefcd3) into master (b782a72) will increase coverage by 0.03%. The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
+ Coverage   88.43%   88.46%   +0.03%     
==========================================
  Files          20       20              
  Lines        1141     1144       +3     
==========================================
+ Hits         1009     1012       +3     
  Misses        132      132              

Impacted Files	Coverage Δ
src/superspawn.js	85.24% <50.00%> (ø)
src/util/plist-helpers.js	76.92% <100.00%> (+1.24%)	:arrow_up:
src/util/xml-helpers.js	99.01% <100.00%> (+<0.01%)	:arrow_up:

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