Fix handling of tokens consisting of separator characters

The parser is no longer hard-coded to skip whitespace. Tokens
such as newlines, whose characters overlap with the separator
characters, can now be correctly recognized.

examples/grammars/javascript.cc

@@ -112,7 +112,7 @@ namespace tree_sitter_examples {
             str("]") }) },
 
         // Keywords
-        { "_terminator", choice({ str(";"), str("\n") }) },
+        { "_terminator", pattern("[;\n]") },
         { "_var", str("var") },
         { "_for", str("for") },
         { "_if", str("if") },