Add libFuzzer support

This adds support for fuzzing tree-sitter grammars with libFuzzer. This
currently only works on Linux because of linking issues on macOS. Breifly, the
AddressSanitizer library is dynamically linked into the fuzzer binary and
cannot be found at runtime if built with a compiler that wasn't provided by
Xcode(?). The runtime library is statically linked on Linux so this isn't a
problem.

test/fuzz/fuzzer.cc

@@ -0,0 +1,27 @@
+#include <string.h>
+#include "tree_sitter/runtime.h"
+
+void test_log(void *payload, TSLogType type, const char *string) { }
+
+TSLogger logger = {
+  .log = test_log,
+};
+
+extern "C" const TSLanguage *TSLANG();
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  const char *str = reinterpret_cast<const char *>(data);
+
+  TSDocument *document = ts_document_new();
+  ts_document_set_language(document, TSLANG());
+  ts_document_set_input_string_with_length(document, str, size);
+
+  TSParseOptions options = {};
+  options.halt_on_error = false;
+  ts_document_parse_with_options(document, options);
+
+  TSNode root_node = ts_document_root_node(document);
+  ts_document_free(document);
+
+  return 0;
+}