Add clang-format rules to enforce include order rules

[vrn-sn]

Changes are in .clang-format and were applied to all our .cpp and .h files in the lute and tests directories (ignoring doctest.h).

The order is:

1. The current source file's header.
2. Any directory-local files like commands.h and generated/* files.
3. All lute/ headers.
4. All Luau/ headers.
5. Luau VM headers (aren't namespaced).
6. Third-party headers like uv.h, curl/, etc.
7. Standard library headers.
8. Other stuff that didn't get matched.