Xtask (#2688)

### Background
Comprehensive Rust requires a number of tools to be installed (e.g.
mdbook and mdbook-course). As mentioned in #2509 (and discussed in
#2469) it would be nice to have a cross platform command for installing
these dependencies. Currently these are installed using a shell script
(`install-mdbook.sh`) but this isn't truly cross platform e.g. for
Windows users.

### Xtask

[xtask](https://github.com/matklad/cargo-xtask) outlines an approach for
automating tasks in a Rust project. It involves using cargo's aliasing
feature to allow us to run commands like `cargo xtask <some task>` to
perform adhoc tasks via a Rust binary that we might otherwise need a
shell script for.

In this PR we add support for a `cargo xtask install-tools` command that
will replace the `install-mdbook.sh` script and install the dependent
tools. We can potentially extend it to support for other tasks e.g.
`cargo xtask fmt`.

---------

Co-authored-by: Eric Githinji <egithinji@google.com>

xtask/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "xtask"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

xtask/src/main.rs

@@ -0,0 +1,69 @@
+use std::{env, process::Command};
+
+type DynError = Box<dyn std::error::Error>;
+
+fn main() {
+    if let Err(e) = execute_task() {
+        eprintln!("{e}");
+        std::process::exit(-1);
+    }
+}
+
+fn execute_task() -> Result<(), DynError> {
+    let task = env::args().nth(1);
+    match task.as_deref() {
+        Some("install-tools") => install_tools()?,
+        _ => {
+            return Err(Box::from(get_help_string(task.as_deref())));
+        }
+    }
+    Ok(())
+}
+
+fn install_tools() -> Result<(), DynError> {
+    println!("Installing project tools...");
+
+    let install_args: Vec<Vec<&str>> = vec![
+        // The --locked flag is important for reproducible builds. It also
+        // avoids breakage due to skews between mdbook and mdbook-svgbob.
+        vec!["mdbook", "--locked", "--version", "0.4.44"],
+        vec!["mdbook-svgbob", "--locked", "--version", "0.2.1"],
+        vec!["mdbook-pandoc", "--locked", "--version", "0.9.3"],
+        vec!["mdbook-i18n-helpers", "--locked", "--version", "0.3.5"],
+        vec!["i18n-report", "--locked", "--version", "0.2.0"],
+        // These packages are located in this repository
+        vec!["--path", "mdbook-exerciser", "--locked"],
+        vec!["--path", "mdbook-course", "--locked"],
+    ];
+
+    for args in &install_args {
+        let status = Command::new(env!("CARGO"))
+            .arg("install")
+            .args(args)
+            .status()
+            .expect("Failed to execute cargo install");
+
+        if !status.success() {
+            let error_message = format!(
+                "Command 'cargo install {}' exited with status code: {}",
+                args.join(" "),
+                status.code().unwrap()
+            );
+            return Err(Box::from(error_message));
+        }
+    }
+
+    Ok(())
+}
+
+fn get_help_string(task: Option<&str>) -> String {
+    if let Some(t) = task {
+        format!(
+            "Unrecognized task '{t}'. Available tasks:
+
+install-tools            Installs the tools the project depends on."
+        )
+    } else {
+        "Missing task. To execute a task run `cargo xtask [task]`.".to_string()
+    }
+}