Jayvee goes ESM (Langium Upgrade)

[georg-schwarz]

Closes #427. Upgrade Langium and switch to ESM as a necessary evil.

• Switch from CommonJS to ESM as Langium 2.0 switched to ESM.
• Most libraries and apps now use esbuild as we found other tools like rollup and vite to not work out of the box with our setup.
• Extract docs generation from docs app into new app docs-generator so that the docs can stay with CommonJS
• Migrated tests to use vitest instead of jest due to ESM compatibility issues.
• Add linter rules to prevent some of the import-related pitfalls we found along the way migrating to ESM.
• Adjusted the docs where necessary.