Add treesitter keymap to reset foldmethod

Seems like treesitter folds get into a confused situation, potentially a
race condition, which is fixed by setting `foldmethod=expr`. This
mapping makes that faster.

lua/plugins/treesitter.lua

@@ -98,5 +98,10 @@ return {
         },
       },
     })
+
+    -- Keymaps
+    vim.keymap.set('n', '<leader>fm', function()
+      vim.o.foldmethod = 'expr'
+    end, {})
   end
 }