Stop mixing control-flow logic and upgrade code base to esm modules

[TimDaub]

Scope

• index.js currently uses the .js extension, but to future proof it we want to use .mjs and convert all imports to esm-based ones
• Additionally, control-flow logic, e.g. "async/await", "promises" and "callbacks" are currently mixed in index.js. But we want to consistently use "async/await"

Deliverables

• Use "fs/promises" and don't use callbacks or ...Sync methods of fs
• Use "async/await" everywhere

Note

For anything express related, using their standard callback logic is fine e.g. app.get(...,...)