Add lint & format checks

[phildenhoff]

• Add Biome to quickly lint & format .ts files
• Use svelte-check to check (not lint) .svelte files
• Use prettier for format .svelte files
• Use cargo to format Rust files
• Use clippy to lint Rust files

And, add a CI pipeline to automatically check formatting on PRs (lint will come later, once I fix all the lint errors)