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.
Pluto is a superset of Lua 5.4 with a focus on general-purpose programming.
`pluto` feature also enables `lua54` feature since they are compatible and share the same API.
Alex Orlenko