Refactor upload and move to typescript

[vanatteveldt]

Edit: move is mostly complete, only upload is left, but that probably needs a bit of refactor first

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I think we should move to type script.

This can be done by installing typescript and renaming sources files to tsx one by one. Imports also need to be changed AFAICS, it doesn't automatically check for a tsx file if you don't specify an extension (?)

Benefits:

• Better automatic documentation, especially of props
• Automatic checking especially of props and callback argument types -> fewer bugs
• Better autocomplete in editor

Process:

• https://webpack.js.org/guides/typescript/ install typescript and ts-loader and set webpack to run tsx files through ts-loader
• change jsx: react to jsx: react-jsx in tsconfig [https://stackoverflow.com/questions/64656055/react-refers-to-a-umd-global-but-the-current-file-is-a-module]
• deal with lots of error messsages

Useful links:

• https://www.typescriptlang.org/docs/handbook/2