Fix ESM

[cmdcolin]

In 5d920d2 we had to revert the ESM-ness of the build back to CJS. This restores it

Larger context

Other ESM modules have trouble importing CJS modules which this module, as noted above, was converted to. This continues on as an ESM

[cmdcolin]

More technical details:

We have a number of 'business needs' including

1. making a modern simple package
2. making our package work in jbrowse-web and other contexts, which does not use LocalFile (node.js fs package)
3. making our package work in jbrowse-desktop, which does use LocalFile (node.js fs package)
4. making our package work with our other data parsers in whatever context that people might use them
5. making our package work in jest, which tends to like commonjs

[cmdcolin]

reverted because of odd build errors in vite embedded "document.head not defined"

[cmdcolin]

actually managed to make work in v2.0.6

[cmdcolin]

there are various challenges with embedded usage, where it complains about use of fs/promises because the LocalFile exclude doesn't work. could revert if it is really troublesome...