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ARROW-4780: [JS] Package sourcemap files, update default package JS version

Change the way we package sourcemaps to make end-consumer builds faster, bump up the default JS version in the main package (it is now widely supported, and all the major build tools will down-compile as necessary). Also adds a "module" entry so we'll be included in https://www.pikapkg.com/blog/introducing-pika-pack/ search results.

Closes https://issues.apache.org/jira/browse/ARROW-4780

Author: ptaylor <paul.e.taylor@me.com>

Closes #3818 from trxcllnt/js/esm-and-sourcemaps and squashes the following commits:

fc6a8595 <ptaylor> create separate sourcemap files to reduce module file sizes and boost consumer compilation times
8019d260 <ptaylor> add "module" entry to package.json for packagers like @pika/web