Support new Luau require system

This commit introduces the `Require` trait that be used to change `require` behaviour.
By default mlua implements behaviour same as `ReplRequirer` in the original Luau.
Unfortunately binary Luau modules are no longer supported by the new system.

src/stdlib.rs

@@ -41,6 +41,8 @@ impl StdLib {
     pub const MATH: StdLib = StdLib(1 << 7);
 
     /// [`package`](https://www.lua.org/manual/5.4/manual.html#6.3) library
+    #[cfg(not(feature = "luau"))]
+    #[cfg_attr(docsrs, doc(cfg(not(feature = "luau"))))]
     pub const PACKAGE: StdLib = StdLib(1 << 8);
 
     /// [`buffer`](https://luau.org/library#buffer-library) library