Upgrade to v0.12.x, remove nvim-treesitter

Switch to tree-sitter-manager.nvim for managing tree-sitter parsers and
queries. Also fixes upgrade issues.

plugin/settings.lua

@@ -89,7 +89,7 @@ vim.opt.joinspaces = false
 -- Set fold level to something high
 vim.opt.foldlevel = 20
 vim.opt.foldmethod = 'expr'
-vim.opt.foldexpr = 'nvim_treesitter#foldexpr()'
+vim.opt.foldexpr = 'v:lua.vim.treesitter.foldexpr()'
 vim.opt.foldtext = ''
 vim.opt.fillchars = { fold = ' ' }