feat: preamble

[jmbuhr]

Refs

#99
#210
#208

[jmbuhr]

@0atman Maybe you can experiment with this with your rust expertise. This PR adds the #!/usr/bin/env -S cargo +nightly -Zscript line at the start of the the rust otter buffer, so a markdown file with

```rust
---cargo
[dependencies]
clap = { version = "4.2", features = ["derive"] }
---
```

```rust
use clap::Parser;

#[derive(Parser, Debug)]
#[clap(version)]
struct Args {
  #[clap(short, long, help = "Path to config")]
  config: Option<std::path::PathBuf>,
}

fn main() {
  let args = Args::parse();
  println!("{:?}", args);
}
```

Results in a rust buffer that looks exactly like the https://doc.rust-lang.org/cargo/reference/unstable.html#script example. See :b2.

However, I'm unable to get rust-analyzer to initialize without a complete cargo project around this.

[0atman]

oh amazing, yeah, let me test this now

[0atman]

I've run this branch locally, and can confirm the same behaviour here.

Reading the Rustaceanvim readme:

rust-analyzer has limited support for standalone files. Many diagnostics come from Cargo. If you're not in a Cargo project, you won't see any Cargo diagnostics.

I think cargo script is a dead end for otter, which is inherently LSP-based. I'm sorry I didn't realise this when I made the suggestion, and thank you for making this feature.

[0atman]

Alternative implementation idea: Could otter accept a specially-formatted rust code block (same as cargo script's ---cargo, and write out an ephemeral Cargo.toml at the same time as the otter.rs files?

I imagine this won't be the only language that would be useful to have such a process in place for a project config file!