Breaking changes and plan forward: drop legacy vim/neovim support

[wookayin]

I have been maintaining a minimal level of backward compatibility support for outdated neovims -- but from now on, there will be no backward compatibility support for old versions of neovim.

This means that neovim will be always assumed to be latest (but not including nightly).

• The minimum neovim version supported will be 0.7.0 and catching up with latest neovim version will be mandatory.
• config/init.vim will be also re-written as config/init.lua
• vimrc will be heavily refactored to (modular) lua configs as much as possible.
• Plugin managers will be switched from vim-plug to packer.nvim. Vanilla vim will have almost no plugins working.
• Overall, these changes will limit the vanilla vim support, but in practice it should have no problem for most of the users.

This is inevitable because all the plugins keep changing and breaking as they require latest features, so there's no point of supporting different versions of neovim using a lot of has('nvim-0.x.x') branchings..