lexer: add Hint to Error, which is expanded as "%s" instead of "%q"

Signed-off-by: Alejandro Mery <amery@jpi.io>

lexer/error.go

@@ -25,6 +25,7 @@ type Error struct {
 	Column   int
 
 	Content string
+	Hint    string
 	Err     error
 }
 
@@ -57,6 +58,10 @@ func (err Error) Error() string {
 		s = append(s, fmt.Sprintf("%q", err.Content))
 	}
 
+	if err.Hint != "" {
+		s = append(s, err.Hint)
+	}
+
 	return strings.Join(s, ": ")
 }