Change tabs with modifier+digit

I have a very strong muscle memory for changing tabs via `⌘+1`, `⌘+2`,
etc.

Even though it’s a little strange in the FIFO tabs model I got used to
it pretty quickly.
sorry last thing on the structure of the comment. It should specify the arguement:
```
// ### shortcut_select_by_index
//
// Keyboard shortcut to select a specific tab, based on its index, not on the tab id
// ```
// @tab_index {number} the tab index [0-9]
// ```
```