git_ui: Add setting to control default click behavior for git panel (#59649)

# Objective

The current behavior of the git panel is that when you click a file it
will open the multi-buffer view that shows all files with changes. If
you use CMD+Click or CTRL+Click, it will open the solo-file experience
where you only see the diff of the file you have selected. But, there is
no way to customize this behavior.

Related to:
https://github.com/zed-industries/zed/pull/56152#issuecomment-4641971669

## Solution

This MR adds a setting that allows you to customize the default click
behavior. The alternate behavior will always be accessible via CMD+Click
or CTRL+Click.

## 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 adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [X] Tests cover the new/changed behavior
- [X] Performance impact has been considered and is acceptable

---

Release Notes:

- Added a setting to control default click behavior for git panel files

---------

Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>

assets/settings/default.json

@@ -1014,6 +1014,11 @@
     //
     // Default: 0
     "commit_title_max_length": 0,
+    // Default action when clicking a changed file in the Git panel.
+    //
+    // Choices: project_diff, file_diff, view_file
+    // Default: project_diff
+    "entry_primary_click_action": "project_diff",
   },
   "message_editor": {
     // Whether to automatically replace emoji shortcodes with emoji characters.