Refactor into standalone plugin

[clason]

Nvim-treesitter 1.0 (expected later this year) will completely remove the module system in order to significantly reduce complexity.

This means that nvim-treesitter-refactor needs to be refactored into a standalone plugin, similarly to https://github.com/nvim-treesitter/nvim-treesitter-context or the indent.lua module in 1.0. (There's no need to hook into nvim-treesitter; refactor methods should simply work if a suitable parser and queries are in runtimepath. This should also allow simplifying the setup as it no longer needs to be shoehorned into the nvim-treesitter setup structure.)