Add UMD bundle

[kcarnold]

I've been trying to help @rpruim make an R package, and one of the challenges that emerged was that some of the runtime environments don't support import(), or need to run offline. See, for example, https://github.com/uwdata/mosaic/pull/929#issuecomment-3495067353

This PR is an attempt at building a self-contained UMD bundle. I don't know if this is even a direction you'd like to go, yet alone whether this approach coheres with your vision of how things should modularize or how the build process should work.

I actually made two different attempts at this; this one has the simpler changes but I've only actually tested my other one: https://github.com/uwdata/mosaic/compare/main...kcarnold:mosaic:add-umd-build?expand=1

Comments welcome.

[domoritz]

ESM should be widely supported and adding another build target is going to incur extra maintenance that I would like to avoid. You can of course still build a bundle for your specific application (or once we have the code in here like we do for the js widget). So I'd vote against adding another build type.

[domoritz]

Closing unless we change our stance on supporting UMD going forward.