fix: output umd build for main key

[keithamus]

I did not notice in #15 but that PR ended up producing a build that only included ES modules.

The main key in a package.json should not be used for files that produce es modules. We should instead produce a umd compatible bundle, and link main to that.

This PR changes the build step to produce both usm and es style modules, and changes the main key to point to the umd build.

[muan]

When we paired it was consciously decided that umd isn't useful to us anymore. Is satisfying what a main field expects the only reason to add it back? Is there anyway to work around that without creating a build file that we don't consume but need to maintain?

[keithamus]

We could omit the main field entirely, and rely only on the module key.

[dgraham]

Something to look forward to is Node's experimental support for a "type": "module" key in package.json. Once that stabilizes, we'll point main to the module file and won't need to build UMD files.

It looks like producing UMD output is fairly easy to do with command line overrides to tsc, so it would be easy to remove when we start using type: module.

[muan]

I don’t have strong opinions against this. Feel free to carry on. Not sure why the tests are failing though.

[koddsson]

This looks good to me 🎇

[muan]

bootstrap.js seems like an over-complication in comparison to loading the native module. Can we keep the test using the module output here?