Switch to typescript

Switch to typescript

- Moves all code to `./code/` to keep the root clean
  - Mainly an aesthetical change for github users, focused on documentation browsing.
  - The reduced root file list should appear less intimidating for non-developers.
- Reorganizes code in logical groups and to avoid circular dependencies.
  - The package splitting could be more granular.
  - Reworks some of the packaging steps and tooling around `tsc` and `rollup`.
  - Generates dependency graphs in `./code/packages/` during build.
- Introduces typescript types in most places.
  - Prefer interfaces, in particular for "dynamically loaded" shared environment code for browsers/`node` and React component classes.
  - Attempts to follow good practices, such as `Readonly<>` types. Linting rules help enforce these good practices.
- Fixes some functional bugs, but apart from that the extension should be the same.
- This big commit consisted of several hundred small(ish) commits, many of which were search-replace steps and iterative typescript/linting fixes. Alas it was not feasible to keep the changes grouped, thus they have all been squashed.