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/main.rs

@@ -0,0 +1,40 @@
+#![allow(clippy::print_stdout)]
+#![allow(clippy::print_stderr)]
+#![allow(clippy::exit)]
+
+mod deny;
+pub(crate) mod utils;
+
+type DynError = Box<dyn std::error::Error>;
+
+fn main() {
+    if let Err(e) = try_main() {
+        eprintln!("{e}");
+        std::process::exit(-1);
+    }
+}
+
+fn try_main() -> Result<(), DynError> {
+    let arg_strings: Vec<_> = std::env::args().skip(1).collect();
+    let args: Vec<_> = arg_strings.iter().map(String::as_str).collect();
+
+    match args.as_slice() {
+        &[] | &["-h"] | &["--help"] => print_help(),
+        &["deny", ..] => deny::deny(&args[1..])?,
+        c => Err(format!("Invalid arguments {c:?}"))?,
+    }
+    Ok(())
+}
+
+fn print_help() {
+    let help = "
+    xtask help
+
+    Subcommands
+    deny: Run cargo-deny for all targets
+
+    Options
+    -h, --help: print help and exit
+        ";
+    println!("{help}");
+}