move highlightjs code into utils directory (#1472)

Following a short discussion on Discord about where to locate developer tools, I suggest we collect developer utils inside a `/utils` directory at the repository root.

The idea behind this directory is taken from LLVM's utils directories in the various LLVM subprojects which host tooling such as Vim and VSCode extensions, utility scripts, and other useful bits for developers developing or using LLVM.

This patch moves the highlightjs code into said utils directory.

Let me know if there's anything else that's missing :smile:

cc @jonmeow

utils/README.md

@@ -0,0 +1,10 @@
+# Developer Utilities
+
+<!--
+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
+-->
+
+This directory collects tools and utilities that may be useful to both Carbon
+developers and developers writing Carbon code.