Highly experimental, unmergeable

This is an experiment to use esbuild. It replaces rollup (and its plugins) TypeScript, Babel and Terser. If we want to try this, I would think about releasing a differently named pipeline first for people who want to try it out. And release it later as a new major version. But let's first figure out, if this is something we want to try.

• https://esbuild.github.io/api/#external
• disregard irrelevant changes
• write error if error occurred
• source map support
• Investigate: how should compiling down to ES6 work?
• Investigate: code splitting
• check if we should still opt-in to jsx and ts in the config file

Things, that we would need to drop support for:

• transpiling for anything below ES6 (higher than that is possible, but not implemented in this PR)
• output format UMD, CJS and IIFE (we could support CJS and IIFE, but I don't know why we would)
• compact option that does not reduce whitespace (broken in main right now anyway)
• exclude, probably