Sync to upstream/release/505 (luau-lang#216)

- Improve error recovery during type checking
- Initial (not fully complete) implementation for singleton types (RFC RFC: Singleton types luau-lang#37)
- Implement a C-friendly interface for compiler (luacode.h)
- Remove C++ features from lua.h (removed default arguments from luau_load and lua_pushcfunction)
- Fix lua_breakpoint behavior when enabled=false
- Implement coroutine.close (RFC RFC: coroutine.close luau-lang#88)

Note, this introduces small breaking changes in lua.h:

- luau_load env argument is now required, pass an extra 0
- lua_pushcfunction now must be called with 3 arguments; if you were calling it with 2 arguments, pass an extra NULL; if you were calling it with 4, use lua_pushcclosure.

These changes are necessary to make sure lua.h can be used from pure C - the future release will make it possible by adding an option to luaconf.h to change function name mangling to be C-compatible. We don't anticipate breaking the FFI interface in the future, but this change was necessary to restore C compatibility.

Closes luau-lang#121
Fixes luau-lang#213

Fixes IFTODT error while compiling from an android device (luau-lang#199

)

Co-authored-by: Arseny Kapoulkine <[email protected]>

Sync to upstream/release/503 (luau-lang#135)

- A series of major optimizations to type checking performance on complex
programs/types (up to two orders of magnitude speedup for programs
involving huge tagged unions)
- Fix a few issues encountered by UBSAN (and maybe fix s390x builds)
- Fix gcc-11 test builds
- Fix a rare corner case where luau_load wouldn't wake inactive threads
which could result in a use-after-free due to GC
- Fix CLI crash when error object that's not a string escapes to top level
- Fix Makefile suffixes on macOS

Co-authored-by: Rodactor <[email protected]>

Update index.md

Fix overlay header - image is not optional :(