Allow opening files from the CLI and from the File > Open menu

[maxbrunsfeld]

This PR allows Zed to open files directly from CLI and from the File > Open menu, without having to use the file finder.

Tasks

• Running zed [files...] opens all of the given files (creating single-file worktrees)
• Selecting a set of files in the File > Open dialog opens all of those files
• The file finder displays single-file worktrees nicely
• The file finder handles multiple files with the same relative path, within different worktrees