Move from yarn to npm, update non-major & tooling dependencies

[Kadrian]

This

• moves back from yarn to npm (the industry has mostly moved over by now + I ran into strange ESM require issues with the latest tooling dependencies using yarn)
• updates all non-major-update dependencies, including react and react-dom
• updates all tooling-dependencies, including typescript and eslint
• changes our eslint-setup, moving away from the old "create-react-app"-eslint-config and over to the "standard" vite / typescript / react eslint-setup – fixing all kinds of types and linting issues along the way – esp: no explicit any anymore.
• cleans up some of the jest dependencies
• replaces the old react-split-pane dependency with react-resizable-panels, because the old one didn't work with the latest react