feat: add better diagnostic rendering

src/frontend/token.rs

@@ -1,4 +1,4 @@
-use std::fmt::Display;
+use std::{fmt::Display, ops::Range};
 
 /// A half-open interval `[start, end)` representing a location in the source text.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -23,6 +23,12 @@ impl Span {
     }
 }
 
+impl From<Span> for Range<usize> {
+    fn from(value: Span) -> Self {
+        value.start..value.end
+    }
+}
+
 /// A fundamental, categorized unit of source code produced during lexical analysis.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct Token<'src> {