Add copy code button to code snippets (#945)

Hello everyone, this is my implementation for the copy button on the snippet (requested in #924)

The implementation is made 100% javascript as with or without a jekyll template modification you still have to execute some javascript code, and I consider it the best choice.

the button only appears if the mouse is over it, to allow the entire line to be read

the important CSS changes were made to make the copy button work even in the long code situation:


![image](https://user-images.githubusercontent.com/26844016/187731472-d4bf7828-2356-4d94-9c2d-9db863228f5f.png)

to avoid this:

![image](https://user-images.githubusercontent.com/26844016/183292313-d7f73d7d-58c0-4c7b-b5ba-e08bd285514b.png

)

Co-authored-by: Matt Wang <matt@matthewwang.me>