improve playground

[mathieudutour]

• remember scripts
• save scripts
• add eslint
• If an error happens in the playground, allow copying it.
• add a plugin command (associated with a shortcut) to run the current script

What else?

[Arkkimaagi]

• If an error happens in the playground, allow copying it.

The idea is to make easier to reference it here, so we do not have to use screenshots and maybe other people can find it with google too.

I was unable to select or copy the error I got in any way on this issue: #27