refactor: change build system, revamp docs & speedup generation

[sgoudham]

This PR aims to give the repository a full overhaul, making it much easier to install and maintain.

TODO:

• Speed up script exponentially. Starting from being unable to build to all flavours/accents in under 4 minutes.
• Store PNGs in the repository
  - The PNGs are currently built everytime into a new build/ directory. This process calls inkscape over thousands of times each time. We should build the PNGs once into the same directory as the SVGs instead of a whole new directory so that the cache mechanism works properly.
• Tidy up makefile/build.sh script, add debug logging, etc.
• Add justfile for nicer developer experience, should aim to be optional so that end users don't need to install it.
• Rename zips to be lowercase, match with new GTK standard.

We still need to figure out a process of recolouring the SVGs fast but it looks
like they were hand-edited and put into source control. This is fine for the
time being and I'm not looking to optimise/improve that process.

BEGIN_COMMIT_OVERRIDE
build: generate zips in lowercase
perf: exponential speed-up in build time
END_COMMIT_OVERRIDE