build(backend): transpile and bundle using esbuild

[hejtful]

Part of: https://linear.app/serlo/issue/PE-128/deployment-of-found-solution-for-serlo-editor-as-lti-tool

Changes:

• When transpiling to CommonJS, import.meta.dirname is unnecessary
• Single script for building frontend and backend, both have their respective directories in dist

Manual tests:

• Still works in local with docker
• Still works in Uberspace

[LarsTheGlidingSquirrel]

Very nice! Thank you!
I only feel like we could merge the two Dockerfiles into one. But we can do that later.