The treesj.lua file is added to the project to enable the toggling of Treesitter Join functionality. The `tsj.setup` function is called to configure the plugin with the option `use_default_keymaps` set to `false`. This ensures that the default keymaps provided by the plugin are not used.
A keymap is set using `vim.keymap.set` to bind the `<leader>m` key combination in normal mode to the command `TSJToggle`. The `opts.desc` option is set to provide a description for the keymap.
This change allows users to toggle the Treesitter Join functionality using the `<leader>m` key combination in normal mode.
