[Demo] Add a justfile, roughly equivalent to the makefile

[dralley]

I learned about this from pysequoia

Just is a command runner, it has many similarities to Make but does a few things differently.
https://github.com/casey/just

The main benefits are:

• just --list shows documentation for commands and groups
• Groups are runnable without needing to define new commands
• No .PHONY boilerplate
• multi-line scripts work properly, so _inner commands are not needed
• dependencies work better (see setup-test-config)
• just commands can be run while in a subdirectory, doesn't have to be the directory of the justfile

Downside is there is no equivalent to the i18n pattern rules so it becomes a script

[mdellweg]

What is the equivalent of the "not .PHONY" targets then?

The nice thing (for me) about make is that it is basically a functional programming language for the filesystem.

[dralley]

I believe it doesn't, because it's a command runner not a build tool. At least I don't see an equivalent, though the docs mention that every recipe in just is equivalent to .PHONY