use Ctrl-n instead of Alt-n on macOS

[zygous]

To use alt-n modifiers on macOS the user must configure their terminal emulator to map option to alt. This isn't obvious -- hence issues have been raised here about those shortcuts not working -- but, moreover, doing so prevents option-3 being used to type # on British Macs.

This change adds a configuration option ctrl_n_shortcuts, set false by default, that replaces the numerical shortcuts alt-n with ctrl-n ones.

This PR only adds ctrl-n support, where n is a number: it does not affect the alt-b or alt-f shortcuts.

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Name	Status	Preview	Comments	Updated (UTC)
atuin	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 17, 2023 7:49pm

[ellie]

Thank you! LGTM.

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