Fix handling of zero-width external tokens at beginning of document

Fixes #316

test/fixtures/test_grammars/epsilon_external_tokens/corpus.txt

@@ -0,0 +1,9 @@
+==========================
+A leading zero-width token
+==========================
+
+hello
+
+---
+
+(document (zero_width))

test/fixtures/test_grammars/epsilon_external_tokens/grammar.json

@@ -0,0 +1,21 @@
+{
+  "name": "epsilon_external_tokens",
+
+  "externals": [
+    {"type": "SYMBOL", "name": "zero_width"}
+  ],
+
+  "extras": [
+    {"type": "PATTERN", "value": "\\s"}
+  ],
+
+  "rules": {
+    "document": {
+      "type": "SEQ",
+      "members": [
+        {"type": "SYMBOL", "name": "zero_width"},
+        {"type": "STRING", "value": "hello"}
+      ]
+    }
+  }
+}

test/fixtures/test_grammars/epsilon_external_tokens/scanner.c

@@ -0,0 +1,33 @@
+#include <tree_sitter/parser.h>
+
+enum TokenType {
+  ZERO_WIDTH_TOKEN
+};
+
+void *tree_sitter_epsilon_external_tokens_external_scanner_create() {
+  return NULL;
+}
+
+bool tree_sitter_epsilon_external_tokens_external_scanner_scan(
+  void *payload,
+  TSLexer *lexer,
+  const bool *valid_symbols
+) {
+  lexer->result_symbol = ZERO_WIDTH_TOKEN;
+  return true;
+}
+
+unsigned tree_sitter_epsilon_external_tokens_external_scanner_serialize(
+  void *payload,
+  char *buffer
+) {
+  return 0;
+}
+
+void tree_sitter_epsilon_external_tokens_external_scanner_deserialize(
+  void *payload,
+  const char *buffer,
+  unsigned length
+) {}
+
+void tree_sitter_epsilon_external_tokens_external_scanner_destroy(void *payload) {}