When neither is specified, if you open a directory you get a new
workspace, otherwise files are added to your existing workspace.

With --new files are always opened in a new workspace
With --add directories are always added to an existing workspace

Fixes #9076
Fixes #4861
Fixes #5370

Release Notes:

- Added `-n/--new` and `-a/--add` to the zed CLI. When neither is
specified, if you open a directory you get a new workspace, otherwise
files are added to your existing workspace. With `--new` files are
always opened in a new workspace, with `--add` directories are always
added to an existing workspace.
([#9076](https://github.com/zed-industries/zed/issues/9096),
[#4861](https://github.com/zed-industries/zed/issues/4861),
[#5370](https://github.com/zed-industries/zed/issues/5370)).