Allow splitting the terminal panel (#21238)

Closes https://github.com/zed-industries/zed/issues/4351


![it_splits](https://github.com/user-attachments/assets/40de03c9-2173-4441-ba96-8e91537956e0)

Applies the same splitting mechanism, as Zed's central pane has, to the
terminal panel.
Similar navigation, splitting and (de)serialization capabilities are
supported.

Notable caveats:
* zooming keeps the terminal splits' ratio, rather expanding the
terminal pane
* on macOs, central panel is split with `cmd-k up/down/etc.` but `cmd-k`
is a "standard" terminal clearing keybinding on macOS, so terminal panel
splitting is done via `ctrl-k up/down/etc.`
* task terminals are "split" into regular terminals, and also not
persisted (same as currently in the terminal)

Seems ok for the initial version, we can revisit and polish things
later.

Release Notes:

- Added the ability to split the terminal panel