Add Ability to Map Alt/Opt Modifier

**Is your feature request related to a problem? Please describe.**

There is currently no support for binding the alt/meta/opt modifier.
I found this out just now when I tried binding it `` and `` to move through tabs.

**Additional context**

`Unrecognized key for keybinding`

Currently supported [keymaps](https://github.com/jesseduffield/lazygit/blob/master/docs/keybindings/Custom_Keybindings.md)