Add jsnext:main support

[dandv]

See jsforum/jsforum#5 and note that while jsnext:main has been superseded by pkg.module (which might get standardized), popular modules like redux and three.js still declare (only) jsnext:main.

[codecov-io]

Codecov Report

Merging #24 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #24   +/-   ##
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  Coverage     100%   100%           
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  Files           1      1           
  Lines           1      1           
  Branches        1      1           
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  Hits            1      1

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[diegohaz]

Thank you, @dandv. I'll take a time to read that issue.

But looking at it, shouldn't we add src to files then? Otherwise src will not be published to npm.

[dandv]

I've just updated the OP - pkg.module has superseded jsnext:main, and module might get standardized.

Indeed, src should be added to files. I'll let you have a look.