(Not fleshing this much out yet until the module is off the ground)
This module is structured as a private NPM package. You should be able to clone it, run npm install, and be on your way.
Development uses npm@8 (included with Node.js 16).
All code is written in TypeScript and lives under "src/". Build output (the installable module) gets copied to a new folder called "module/".
Strings (strings/en.json) live in the src/ directory, and are automatically copied into the module directory by the TypeScript compiler.
The module manifest (module.json) is not copied automatically (this happens in a GitHub workflow). If you need it during dev, you'll have to copy it manually for now.
This package uses a combination of foundry-pc-types and custom type definitions where foundry-pc-types uses any or is otherwise missing concrete definitions.
First, build your changes (npm run build); this will generate an installable module in the "module" directory.
Then either copy the folder into your Foundry VTT "modules/" data folder (renamed "hidden-intiative/"), or create a symlink called "hidden-initiative/" from the modules directory to your local module directory.
Pull requests into master are welcome and I will review them if they show up.