Add select-all and invert-selection to all select-like prompts

[aqeelat]

Context

This came up while evaluating @clack/prompts as a potential replacement for enquirer.js, comparing it with enquirer.js and @inquirer/prompts.

Split out from: #550

Problem

The flat multiselect prompt supports:

a = toggle all
i = invert selection

groupMultiselect does not currently support the same keybindings across all groups.

This makes bulk selection harder in grouped prompts, especially when there are many options.

Proposal

Add select-all and invert-selection behavior to all relevant select-like prompts.

For groupMultiselect:

• a should toggle all selectable options across all groups
• i should invert all selectable options across all groups
• Disabled/separator/header rows should not be affected

This would make grouped selection behavior more consistent with multiselect.