workspace: Open new projects in the same window by default (#54057)

This makes it so that `cmd-o` adds the project to the multi-workspace,
rather than opening a new window

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)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

crates/workspace/src/workspace.rs

@@ -231,7 +231,7 @@ pub struct Open {
 
 impl Open {
     pub const DEFAULT: Self = Self {
-        create_new_window: true,
+        create_new_window: false,
     };
 
     /// Used by `#[serde(default)]` on the `create_new_window` field so that