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/mappings.lua

@@ -59,7 +59,7 @@ vim.keymap.set('n', '<leader>gw', function()
 end, { desc = 'Format whole file with gw' })
 
 vim.keymap.set('n', '<leader>fm', function()
-  if vim.o.foldexpr == 'nvim_treesitter#foldexpr()' then
-    vim.opt.foldexpr = 'nvim_treesitter#foldexpr()'
+  if vim.o.foldexpr == 'v:lua.vim.treesitter.foldexpr()' then
+    vim.opt.foldexpr = 'v:lua.vim.treesitter.foldexpr()'
   end
 end)