Fix crash w/ goto_previous_sibling when parent node has leading extra child (#4472)

* Fix crash w/ goto_previous_sibling when parent node has leading extra
child Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

* Fix lint

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

test/fixtures/test_grammars/aliases_in_root/corpus.txt

@@ -0,0 +1,13 @@
+======================================
+Aliases within the root node
+======================================
+
+# this is a comment
+foo foo
+
+---
+
+(document
+  (comment)
+  (bar)
+  (foo))

test/fixtures/test_grammars/aliases_in_root/grammar.js

@@ -0,0 +1,19 @@
+module.exports = grammar({
+    name: 'aliases_in_root',
+
+    extras: $ => [
+      /\s/,
+      $.comment,
+    ],
+
+    rules: {
+        document: $ => seq(
+          alias($.foo, $.bar),
+          $.foo,
+        ),
+
+        foo: $ => "foo",
+
+        comment: $ => /#.*/
+    }
+});