Add Haunt for Scheme as a provider

[Siarune]

Creates and registers a build provider for Scheme with the static builder Haunt.
Adds a very basic example to test provider.
Adds docs

[coffee-cup]

Looks awesome! Just a few minor comments

[github-actions]

This pull request is stale because it has been open 10 days with no activity. Remove stale label or comment or this will be closed in 5 days

[Siarune]

My build plan commands seem out of order, and I can't tell why.

This is my plan definition:

let setup = Phase::setup(Some(vec![Pkg::new("haunt")]));
let build = Phase::build(Some("haunt build".to_string()));
let start = StartPhase::new("haunt serve".to_string());

let plan = BuildPlan::new(&vec![setup, build], Some(start));
Ok(Some(plan))

So this is what I'd expect the order to be:

╔════ Nixpacks v1.29.0 ════╗
║ setup      │ haunt       ║
║──────────────────────────║
║ build      │ haunt build ║
║──────────────────────────║
║ start      │ haunt serve ║
╚══════════════════════════╝

But this is what it actually is

╔════ Nixpacks v1.29.0 ════╗
║ build      │ haunt build ║
║──────────────────────────║
║ setup      │ haunt       ║
║──────────────────────────║
║ start      │ haunt serve ║
╚══════════════════════════╝

[coffee-cup]

You can specify phase dependencies. By default the build phase depends on the install phase, but if there is no install you need to specify the dependency manually.

build.depends_on_phase("setup")

[github-actions]

This pull request is stale because it has been open 10 days with no activity. Remove stale label or comment or this will be closed in 5 days