Add a keyboard shortcut to expand a pane to full width

First off, the pane shortcuts are great and I use them constantly — ⌘⇧B for
the browser, ⌘J for the terminal, ⌘⇧D for the diff.

The gap is that they only open and close a pane. Once it's open there's no
way to make it bigger without grabbing the mouse and dragging the divider.

That's the part that breaks for me. I hit the shortcut to bring up the
browser, and then immediately have to reach for the mouse to actually see
anything in it. Every time.

What I'd love is a second keypress. One shortcut opens the pane, another
expands it to the full width of the window. Hit it again and it goes back.
That's all.

I went looking at what the View menu can actually send, and the pane
commands are only toggleSessionPane, closeSessionPane and splitViewCommand
— there's nothing for sizing. So I think this needs a new action, not just
a new binding.

The closest I can get today is hiding the sidebar (⌘B) and the side chat
(⌘;) to free up space. It helps, but that's three shortcuts and still not
full width.

Would really love to see this one land. macOS, Code tab.

Start by tracing the View-menu actions toggleSessionPane, closeSessionPane, and splitViewCommand in the macOS Code tab, then locate the pane sizing and shortcut entry points. Done means a new action can expand the pane to full window width and toggle it back without dragging, with the existing pane shortcuts still working.

Written by the indexing model from the issue text.