Add support for debug vs release configurations to the Ninja build process

[rdw-software]

No need for fancy things, just a simple flag that can be passed to the build script: unixbuild.sh debug (and default to release)

It's trivial to detect this in Lua (global arg) and then swap in a few different flags (if preset == "debug" then ...).

[rdw-software]

Instead of hacking even more complex logic into what's supposed to be a tiny utility script, maybe there's a simpler way:

• Define presets with ENV files (similar to the one I just now created for the autoformat workflow)
• Provide a set of standard presets that can be used in build workflows directly (e.g., debug.env, release.env)
• Allow passing the name of a custom preset (ENV file) as argument to override those defaults (e.g., custom.env)
• This would be similar in functionality to the typical Makefile approach, but much less arcane and easier to maintain
• The same method can be used to modify build options for the embedded libraries and create variants at "zero cost"

It's possible there are some limitations that I didn't think about, but it's promising enough to warrant some experimentation.