Migrate to esm

[cloverich]

As I implemented #117 I ran into some issues w/ esm. I attempted to migrate the project, but failed. I suspect it may have partially been mis-configuring. At any rate, here's a quick summary of what I learned...

• ESM was designed for browsers; node adoption came later, but is the future
• There are at least four relevant tsocnfig.json settings
• Esbuild also plays a role: When bundles target node (my preload and main script), it emits commonjs

For tsconfig:

• target: this just controls things like polyfills, i.e. "if I use fancy syntax, its valid / browser will provide it"
• module: Whether to emit as commonjs or esm (+ a ilttle more nuance). What THIS app will be.
• esModuleInterop: Helpers to handle imports that may be one or the other. What the DEPENDENCIES are.
• moduleResolution: Yup, I've already forgotten

Further once compiled (typescript), and bundled (esbuild), there's an additional factor of Node (or the browser) being able to run the thing; the file extensions must be .mjs for the prebuild script (I think); package.json having type: "module" tells the node process to run as esm.

ANYWAYS, lots of moving parts. I may have to upgrade some of my dependencies; because of that I think it makes more sense to knock out the gonna-happy-anyway-dependency changes, likely as part of:

• Review yarn vs npm vs others #137
• Design hackathon #129 (probably I'll drop some UI dependencies here)
• Migrate from mocha to node's built-in testing library