Migrate from JavaScript --> TypeScript

[spwoodcock]

Is your feature request related to a problem? Please describe.

• Currently the codebase is JavaScript.
• It's a bit messy as we do have some TypeScript declarations inside JS files...
• We probably need a big refactor to improve the maintainability of the codebase.
• Other issues such as Replace craco / webpack frontend module bundler with vite #6531 are partly blocked by this.

Describe the solution you'd like

• Rename .js files to .jsx.
• Upgrade all code JavaScript --> TypeScript, with js and jsx to ts and tsx.

Describe alternatives you've considered

• JSDoc, but IMO its better for installable packages, not for software tools, where typescript shines.

Additional context

• This is low priority, but will be helpful into the future.
• @JoltCode has kindly volunteered to look into this.
• It's a minor blocker for some other tasks, as it makes sense to do this first.