VS Code has a command called "Toggle Maximize Editor Group" (`Cmd+K
Cmd+M`) that expands the active editor pane to fill the entire center
area, hiding editor split panes, but leaving docks and panels visible.
Zed's existing `ToggleZoom` hides all panels and is closer to VS Code's
Zen Mode. VS Code has both — it would be useful for Zed to as well.

This is particularly useful for people working on laptops, and has been
raised before (#32715, #4897, #32860), but the addition of the project
panel alongside the agent panel has made it more relevant. I work with
the editor split vertically, but with the project and agent panels both
visible, on a laptop screen there's really only room for a single editor
pane. This command makes it easy to switch to and from that mode.

**Behavior:**
- New action: `workspace::ToggleEditorZoom`
- When activated, the active editor pane expands to fill 100% of the
center/editor area, hiding sibling split panes
- Docks and panels remain visible and unaffected
- Toggling again restores the previous split layout
- If `ToggleZoom` is active, it unzooms first, then maximizes

Here's a demo:


https://github.com/user-attachments/assets/fcf6748e-6c45-4c4a-8d9f-f106c6cd58ea

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #32715

Release Notes:

- Added `workspace::ToggleEditorZoom` action to maximize the active
editor pane within the center area while keeping panels visible.