Keybind disable mode

# Description of the new feature/enhancement

There should be a way to allow "passthrough" of keybindings to the terminal session - for example, the keybinding Ctrl+Shift+↓ is used in Terminal to resize a pane, but it's also used by the Linux terminal multiplexer BYOBU to resize panes.

I envision something like a command in the palette to disable Terminal keybindings and allow then to pass through to the terminal itself. This command would automatically disable "focus mode" (since there'd be no way to exit focus mode again) and would be a toggle.

I searched around in existing open/closed tickets as well as the documentation to see if something like this already exists, and as far as I can tell it doesn't. #5790 is somewhat related, in that it could be a "workaround" for this feature.

# Proposed technical implementation details (optional)

I imagine it'd be a fairly simple matter to have an early exit in the keybind handling code to check if this mode is enable, and if so act as if there are no keybindings.

Start by locating the keybind handling code and the command palette entry points referenced by the issue. Trace how focus mode is enabled and disabled before deciding where the toggle belongs. Done means a palette command can toggle terminal-keybinding passthrough, disables focus mode, and lets keys such as Ctrl+Shift+Down reach the terminal session.

Written by the indexing model from the issue text.