Remove Svelte v2 support

[TheComputerM]

I do not think anyone is using this lib with Svelte v2 anymore, so I suggest to remove support for it as it is just additional maintenance and bloat for almost all of the users.

[alexprey]

Hi @TheComputerM,
Just talk about that with other developers and I think it is a good point and we can start with clear infrastructure on new major version 5.X