Add xtask crate (#4293)

Replaces only the cargo_deny.sh script for now. Can be expanded over
time to replace the other shell and python scripts, so only Rust is
needed to work with the repository.

Closes <https://github.com/emilk/egui/issues/2887>
Closes <https://github.com/emilk/egui/issues/4373>

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

xtask/src/utils.rs

@@ -0,0 +1,45 @@
+use std::{
+    env,
+    io::{self, Write as _},
+    process::Command,
+};
+
+use super::DynError;
+
+/// Print the command and its arguments as if the user had typed them
+pub fn print_cmd(cmd: &Command) {
+    print!("{} ", cmd.get_program().to_string_lossy());
+    for arg in cmd.get_args() {
+        print!("{} ", arg.to_string_lossy());
+    }
+    println!();
+}
+
+/// Prompt user before running a command
+///
+/// Adapted from [miri](https://github.com/rust-lang/miri/blob/dba35d2be72f4b78343d1a0f0b4737306f310672/cargo-miri/src/util.rs#L181-L204)
+pub fn ask_to_run(mut cmd: Command, ask: bool, reason: &str) -> Result<(), DynError> {
+    // Disable interactive prompts in CI (GitHub Actions, Travis, AppVeyor, etc).
+    // Azure doesn't set `CI` though (nothing to see here, just Microsoft being Microsoft),
+    // so we also check their `TF_BUILD`.
+    let is_ci = env::var_os("CI").is_some() || env::var_os("TF_BUILD").is_some();
+    if ask && !is_ci {
+        let mut buf = String::new();
+        print!("The script is going to run: \n\n`{cmd:?}`\n\n To {reason}.\nProceed? [Y/n] ",);
+        io::stdout().flush().unwrap();
+        io::stdin().read_line(&mut buf).unwrap();
+        match buf.trim().to_lowercase().as_ref() {
+            "" | "y" | "yes" => {}
+            "n" | "no" => return Err("Aborting as per your request".into()),
+            a => return Err(format!("Invalid answer `{a}`").into()),
+        };
+    } else {
+        eprintln!("Running `{cmd:?}` to {reason}.");
+    }
+
+    let status = cmd.status()?;
+    if !status.success() {
+        return Err(format!("failed to {reason}: {status}").into());
+    }
+    Ok(())
+}