Migrate nvim-treesitter to neovim-treesitter when moving to nvim 0.12+.

The [nvim-treesitter plugin]([https://github.com/nvim-treesitter/nvim-treesitter) has been archived, due to developer burnout, and won't be updated to support versions 0.12 and beyond of neovim. 

There is an [active fork](https://github.com/neovim-treesitter/nvim-treesitter) that is being maintained by other users - switch to this one when moving to neovim version 0.12 or greater.