feat(menu): add MenuAccept to take a completion without submitting

[kronberger-droid]

Summary

Enter over an open menu accepts the highlighted item, but the same key submits the line once no menu is open,
so there is no event a user can bind to "accept the completion, then keep typing".
Two stale PRs ran into that from different sides:
#965 wanted a zsh-style space that accepts and types itself and had to invent SubmitOrSpace for it,
#821 wanted an always-open menu and had to invent its own MenuAccept so Enter could stay free for submit.

This is the small shared piece: ReedlineEvent::MenuAccept.
It does what Enter does while a menu is open, splice the selection and deactivate, and stops there.
It reports Inapplicable when no menu is active or the active one has no values,
so it composes with UntilFound and Multiple like the other menu events.

New ReedlineEvent variant, thus a public API addition. No default keybinding changes and no observable change for anyone not binding it.

Before

Accepting a menu item is only reachable through Enter/Submit, which submit the buffer when the menu happens to be closed.

After

keybindings.add_binding(
    KeyModifiers::NONE,
    KeyCode::Char(' '),
    ReedlineEvent::Multiple(vec![
        ReedlineEvent::MenuAccept,
        ReedlineEvent::Edit(vec![EditCommand::InsertChar(' ')]),
    ]),
);

Space accepts the highlighted completion and types itself, and is a plain space when no menu is open.
An empty menu is left alone rather than closed, so the keypress is not spent on a menu with nothing in it,
which is the same direction #1175 takes the Enter arm.

Additional notes

Supersedes #965. First piece of #821, tracked in #356.