Fix empty keybind setting

[unknown]

Fixes #5936.

Extracts the default keybind setting to an init function. Add logic to call init in parseIntoField if it is defined for the type.

[mitchellh]

One easy request.

A couple notes for the future (we should NOT do them in this PR, just to keep things simple, but can follow up):

1. We should move the keybindings out to a dedicated file. They've been causing too much noise in Config.zig for too long and this reminded me. src/config/keybinding.zig or something.

2. We should make a src/cli/args.init function that takes a struct and uses comptime to find all the fields that have an init and call them so that we have one place we can always use to default any "config" structure (we use args for other stuff too).

[mitchellh]

Apologies, one small thing. I'd fix it myself but not at a PC rn.

[mitchellh]

Perfect. Thanks!

[unknown]

Apologies, one small thing. I'd fix it myself but not at a PC rn.

Perfect. Thanks!

No worries! I can work on the followups when I get the chance.