ast: formatting; main: explicit verbosity

src/main.rs

@@ -205,7 +205,10 @@ fn subst() -> Result<(), Box<dyn Error>> {
 
 fn parse(document: &str) {
     let mut parser = Parser::new(Lexer::new(document));
-    let isatty = std::io::stdin().is_terminal();
+    let verbose = !matches!(
+        std::env::var("DOUGHLANG_VERBOSE").as_deref(),
+        Ok("false" | "0" | "no")
+    );
     for idx in 0.. {
         match parser.parse::<Expr>(0) {
             Err(e @ ParseError::EOF(s)) if s.tail == document.len() as _ => {
@@ -220,7 +223,7 @@ fn parse(document: &str) {
                 println!("\x1b[91m{e}\x1b[0m");
                 break;
             }
-            Ok(v) if isatty => {
+            Ok(v) if verbose => {
                 println!("\x1b[{}m{v}", (idx + 5) % 6 + 31);
             }
             _ => {}