Export library as both UMD and ESM

[Jordan-Lane]

This PR adds support to export the library in both UMD and ESM format.

Now the build system exports UMD format (index.cjs), ESM format (index.mjs), and types at the root level. Exporting types at the root build level allows IDEs such as VSCode to find the typings when three-loader is used as a git submodule.

[Jordan-Lane]

I originally closed as I thought I encountered an issue with the UMD build, however it was due to another local change. Reopening this as I think it is still a good starting point.

I am using Vite for my build system, so I have been able to verify the ESM build, however have had some difficulty verifying that the existing UMD build works as expected. Would someone from pnext be able to verify the UMD build works with their existing system?

If not, I can dig into the UMD build more, however might be easier verifying with pnext's original systems.

[Jordan-Lane]

Related issues: #15 and #67

[pjurczynski]

We've run prettier recently. It's highly recommended to first run "npm run format:write", commit and rebase.