Add a separate code scope for plugin install/update

[Gamer-Kold]

There are several kakoune plugins that require compiling them from some sort of source, (such kak-lsp and kak-dap). So in order to install the plugins: you have to compile it and move to PATH, or have it done for you by a command such as "cargo install". However, it seems a bit much to have to do this every time you load up kakoune; when it really only has to be done when the plugin needs updating. It would be great if it were possible to configure commands to only run when plugins are installed/updated not when loaded.