feat: enable switching to the next and previous float terminal

[Blackmorse]

TL;DR

Introducing of ToggleTermFloatNext/ToggleTermFloatPrev commands for the float terminals. Just switching to the next/previous terminal id from the list in a cycled manner.

Motivation

Since I usually use float windows for the terminals, in my flow I'm missing the feature of "switching to the previous/next terminal".

E.g. It's common for me to have 3 terminals at the same time, and switching between them with the count feature is not very convenient, as I have to keep in mind some kind of mapping of the terminal number to its actual state. I'd like to switch between them just by going forward/backward

After Neovim 0.9 release and together with this feature (https://github.com/akinsho/toggleterm.nvim/issues/363) process of switching will become even more transparent for the user.

It's my first experience in Lua/Neovim development, so don't judge me too much, I will be happy to fix any notes and issues :)