Support for broadcasting input to tabs, not just panes

# Description of the new feature/enhancement

I'd like a way to broadcast keyboard input to multiple tabs at once.

This has been asked for across multiple requests and they all get marked as a duplicate of #2634 but that only covers doing broadcasts in panes. This is a different request and I was unable to find an open issue for this. There are many examples of requests that are marked as duplicates of that but are asking for support for tabs, not panes.

# Proposed technical implementation details (optional)

I don't have a preference for how this is done visually, just that I'm able to broadcast to multiple tabs via some mechanism. This can be via a new input window or (like iTerm2 does it) just sending exactly what I send in one tab to other tabs. Ideally there would be some options for choosing which tabs to which you're broadcasting.

No files, tests, or entry points are named. Start by comparing the requested tab behavior with the pane-only scope of #2634 and locate the existing input-broadcasting path; done means users can choose target tabs and send one tab’s keyboard input to them.

Written by the indexing model from the issue text.