Fix CRLF handling in external scanner

corpus/crlf.txt

@@ -0,0 +1,13 @@
+================================
+Variables with CRLF line endings
+================================
+
+A=one
+
+B=two
+
+---
+
+(program
+  (variable_assignment (variable_name) (word))
+  (variable_assignment (variable_name) (word)))

src/scanner.cc

@@ -213,6 +213,7 @@ struct Scanner {
         if (
           lexer->lookahead == ' ' ||
           lexer->lookahead == '\t' ||
+          lexer->lookahead == '\r' ||
           (lexer->lookahead == '\n' && !valid_symbols[NEWLINE])
         ) {
           skip(lexer);