Provide `__tostring` metamethod for built-in library functions that return userdatas

[hello-42]

In the past, built-in library APIs like net.request returned a table that could be pretty-printed. Now it returns a userdata in the form of <(Response)> when printed. In the spirit of improving Lune's developer experience, I think userdatas that are returned from specific built-in library functions should be encoded and decoded internally when __tostring() is called via print(userdata).

Something valuable to note:
When called, __tostring() will cause errors if the returned value's type isn't string.

I'm not sure what this means for the implementation of pretty-printed tables from userdatas, since you'll have to encode a table to a string and then decode the desired data structure back to a Luau table in order to pretty print it. My hope is there's some funny business that can be done internally to remedy that.