Add Lua.pushExternalString - #237

@robbielyman I think this one is pretty straightforward. Any thoughts before I merge?

Looks good to me! One question I had: since the Lua 5.5 function takes a length parameter, would it be better to allow []const u8 instead of [:0]const u8?

The reference manual mentions the string should have a zero at the end, so I think Lua requires it to be null terminated for some reason

Ahh ok. I guess it is true that toString() is supposed to return a null-terminated string, even when you use the variant that takes a length parameter, so I'd guess it's probably for that.

If that's the case, it looks good to merge to me! I just notice the checkSupport function that must have been added in the past couple months too, love that.