Use `eleventy-rollup-plugin` to build the editor at build time.

Right now, the editor is built separately via rollup on the command line in `npm run build:editor`. It would be much nicer to build it via this plugin:
https://github.com/Snapstromegon/eleventy-plugin-rollup

Doing so should also mean that `editor.mjs` changes should get watched by the eleventy watch system--which would also avoid some of the current build tedium.