workspace: Do not reuse window for sub directory (only for root directory and sub files) (#24560)

Closes #10232

Context:

We have three ways to open files or dirs in Zed: `zed`, `zed --new`, and
`zed --add`. `--new` forces the project to open in a new window, while
`--add` forces it to open in an existing window (even if the dir isn’t a
subdir of an existing project or the file isn’t part of it).

Using just `zed` tries to open it in an existing window based on similar
logic of `--add`, but if no related project is found the dir, opens in a
new window.

Problem:

Right now, subdirs that are part of an existing project open in the
existing window when using `zed`. By default, subdirs should open in a
new window instead. If someone wants to open it in the existing window,
they can explicitly use `--add`. After this PR, only root dir and files
will focus on existing window, when `zed ` is used.

Fix:

For the `zed` case, we’ve filtered out subdirs in the logic that assigns
them to an existing window.

Release Notes:

- Fixed an issue where subdirectories of an already opened project, when
opened via the terminal, would open in the existing project instead of a
new window.