build(rust): vendor rnix parser crate

third_party/crates/rnix-0.14.0/examples/from-stdin.rs

@@ -0,0 +1,13 @@
+use std::{io, io::Read};
+
+fn main() {
+    let mut content = String::new();
+    io::stdin().read_to_string(&mut content).expect("could not read nix from stdin");
+    let ast = rnix::Root::parse(&content);
+
+    for error in ast.errors() {
+        println!("error: {}", error);
+    }
+
+    println!("{:#?}", ast.tree());
+}