Treesitter parser (#2902)

A push towards better editor support.
See utils/treesitter/README.md

---------

Co-authored-by: josh11b <josh11b@users.noreply.github.com>

utils/treesitter/test_runner.cpp

@@ -0,0 +1,80 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <tree_sitter/api.h>
+#include <tree_sitter/parser.h>
+
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <sstream>
+#include <string>
+#include <vector>
+
+extern "C" {
+TSLanguage* tree_sitter_carbon();
+}
+
+// Reads a file to string.
+static auto ReadFile(std::filesystem::path path) -> std::string {
+  std::ifstream file(path);
+  std::stringstream buffer;
+  buffer << file.rdbuf();
+  file.close();
+  return buffer.str();
+}
+
+// TODO: use file_test.cpp
+auto main(int argc, char** argv) -> int {
+  if (argc < 2) {
+    std::cerr << "Usage: treesitter_carbon_tester <file>...\n";
+    return 2;
+  }
+
+  auto* parser = ts_parser_new();
+  ts_parser_set_language(parser, tree_sitter_carbon());
+
+  std::vector<std::string> failed;
+  std::vector<std::string> skipped;
+  for (int i = 1; i < argc; i++) {
+    std::string file_path = argv[i];
+    std::string source = ReadFile(file_path);
+
+    // `and` in where clauses is not parsed correctly.
+    // TODO: remove once where clause is implemented correctly.
+    if (source.find("where") != std::string::npos &&
+        source.find("and") != std::string::npos) {
+      skipped.push_back(file_path);
+      continue;
+    }
+    auto* tree =
+        ts_parser_parse_string(parser, nullptr, source.data(), source.size());
+
+    auto root = ts_tree_root_node(tree);
+    auto has_error = ts_node_has_error(root);
+    char* node_debug = ts_node_string(root);
+
+    std::cout << file_path << ":\n" << node_debug << "\n";
+    if (has_error) {
+      failed.push_back(file_path);
+    }
+
+    free(node_debug);
+    ts_tree_delete(tree);
+  }
+  ts_parser_delete(parser);
+  for (const auto& file : skipped) {
+    std::cout << "SKIPPED " << file << "\n";
+  }
+  for (const auto& file : failed) {
+    std::cout << "FAILED " << file << "\n";
+  }
+  if (!skipped.empty()) {
+    std::cout << skipped.size() << " tests skipped.\n";
+  }
+  if (!failed.empty()) {
+    std::cout << failed.size() << " tests failing.\n";
+    return 1;
+  }
+}