Add keybindings for cycling through overloads in signature help

[ural89]

I am using CTRL + J and CTRL + K keys for cycling through anywhere in the editor. So I need this to be add keybinding for signature overloads. "auto_signature_help": true this should be true by default imo.

[SomeoneToIgnore]

zed/assets/keymaps/default-linux.json

Lines 780 to 786 in a05358f

	{
	"context": "Editor && showing_signature_help && !showing_completions",
	"bindings": {
	"up": "editor::SignatureHelpPrevious",
	"down": "editor::SignatureHelpNext"
	}
	},