Improve error message in lexer (#242)

In the error message for an invalid character, print which character
triggered the error.

src/syntax/lexer.rs

@@ -512,7 +512,7 @@ impl Lexer<'_> {
 
             c if is_id_start(c) => self.ident(start),
 
-            _ => self.error("this character is not valid in code"),
+            c => self.error(eco_format!("the character `{c}` is not valid in code")),
         }
     }