Add general api

[makspll]

Closes #5

Todo:

• Move to bevy main branch for ReflectDefault
• Complete entity/component rlua::UserData implementations
• Automatic support for bevy Reflect types
• Implement a base API for Rlua
• Do the same things but for Rhai this will be done in another release

[makspll]

After wrestling with this for a while, it would be very hard to pull off in bevy 0.7.
Biggest missing piece is the ability to initialise components via their Default implementation through Reflect

[coderedart]

isn't it better to use mlua rather than rlua? mlua is much more actively maintained fork of rlua.
https://github.com/lenscas/tealr also has inbuilt features for documentation.

[makspll]

@coderedart
I was mostly influenced by this article: https://www.boringcactus.com/2020/09/16/survey-of-rust-embeddable-scripting-languages.html when choosing between all the lua implementations, which seemed to indicate issues using it on Windows and hence I choose rlua. However I realise that may not be the best choice, tealr seems to be a great argument for mlua since I would like to eventually be able to automatically generate documentation. Swapping to mlua should not be very difficult however since afaik the interface is really close.

Actually, I may be able to even swap between mlua/tealr with a cargo feature.