Remove jsnext:main support from repo

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• Rollup Plugin Name: node-resolve, all others
• Rollup Plugin Version: n / a

Expected Behavior / Situation

n / a

Actual Behavior / Situation

After @TrySound mentioned that jsnext:main is deprecated, I did some digging and asking around. Rollup was really the only bundler that embraced it fully. As mentioned by Rich Harris here, jsnext:main has been superseded by module, and there are numerous sources that consider it deprecated as a result (for example).

Basically it never made it out of discussion, and Rollup supported it out of necessity at the time it was implemented.

Modification Proposal

So all that goes to say is that we have sufficient evidence to remove support from @rollup/node-resolve and to remove that field from any plugins in the repo.

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#85 took care of most of the work. Going to leave this open as several modules have to get a version bump and get published.