On Backspace and MoveToLineStart, deactivate menu and do not complete

[dandavison]

Draft PR, fixes https://github.com/nushell/nushell/issues/5497

As described in https://github.com/nushell/nushell/issues/5497 and in Discord #completions Backspace is causing the completions menu to be updated when it should not (the user is simply deleting; they're not asking for completions of the shorter input). Furthermore MoveToLineStart does something similar, but even more obviously incorrect, and sometimes the menu is rendered incorrectly (see screenshots in Discord #completions).

This PR changes things so that Backspace, BackspaceWord and MoveToLineStart deactivate the completions menu, instead of triggering completions of the new, shorter, input.

(The diff is mostly indentation changes)