fix: use token.immediate() on end string quote

**Problem:** The end quote on a string was not marked as an immediate
token, so trailing whitespace after `string_content` would not be
captured by `string_content`, and would just be part of the `string`
node, with no child.

**Solution:** Mark the end delimiter as an immediate token, meaning all
trailing whitespace gets captured as `string_content`.

Author: ribru17

grammar.js

@@ -72,7 +72,7 @@ module.exports = grammar({
     string: $ => seq(
       '"',
       optional($.string_content),
-      '"',
+      token.immediate('"'),
     ),
     string_content: $ => repeat1(choice(token.immediate(prec(PREC.STRING, /[^"\\\n]+/)), $.escape_sequence)),
     parameters: $ => repeat1(choice($.capture, $.string, $._node_identifier)),