Don't allow registry to be set to readonly

[LoganDark]

Roblox does not do this, but I know for a fact that doing this causes Luau to crash and burn almost instantly (it's fun to use it to crash Roblox even). It would be nice to have some protection against doing it by accident.

[zeux]

This should be an api_check as it's a contract for embedding code, and having lua_setreadonly throw from the static analysis perspective wouldn't be great for code flow analysis.

[LoganDark]

This should be an api_check as it's a contract for embedding code, and having lua_setreadonly throw from the static analysis perspective wouldn't be great for code flow analysis.

Done

[zeux]

Thanks!