gh stack submit: Add tabbing to a Submit button as alternative to Control-S (XOFF flow control) keyboard shortcut

[bhamiltoncx]

Requiring Control-S to submit a stack from the interactive UI is not very ergonomic (not everyone can stretch these fingers easily on every keyboard). It's also the XOFF flow control stop sequence, which can cause freezes in terminals if cooked mode is not disabled.

Please add an alternative (e.g., tabbing to a Submit button) to Control-S to submit a stack.