UMD builds

[luxaritas]

Right now, our builds work as follows:

• If in app mode, bundle, if in lib mode, don't bundle and externalize all dependencies
• Compile both ESM and CJS output

However, this precludes distributing web/iso libraries as semi-contained UMD bundles. In order to do this, we would need to re-run vite build multiple times (ie, in the build script, presumably setting some environment var, disabling emptyOutDir for the later run(s)). It might look something like:

1. Build "dev" ESM + CJS, unbundled, fully external dependencies
2. Build UMD, bundled source, bundled dependencies
3. Build UMD, bundled source, externalize only some specified dependencies (if specified - eg, we could make a build of a Vue library without Vue, assuming the consumer would already have it installed so we don't have to ship it again)

[luxaritas]

It's worth considering whether or not we want to take on the overhead of UMD builds - you can import ES modules via CDN (Vue includes instructions in their docs for doing that with Vue). However, we may still want to provide fully bundled/partially bundled ESM builds (and at that point, maybe UMD isn't much of a lift?). Eventually, it would be great to not even export to CJS, but I'm not sure if adoption is sufficiently high enough yet.