Allow `--wait` flag to work with split editors

Type: <b>Bug</b> This is a reopening of #150368, which was not interpreted as a bug. It is a bug. The `--wait` or `-w` flag behavior is unexpected and probably broken in combination with split editors. The `--help` output says this: > Wait for the files to be closed before returning. This description is ambiguous in relation to split editors. In fact the implementation doesn't do the "common sense" thing, which is to wait for the user to be *done editing*, but instead waits for a "lowlevel" editor closure event, which isn't useful behavior. The only trustable signal that the user is done, is when they have closed *all* editor instaces of the file(s) supplied from the command line. This unexpected behavior can lead to genuine problems, even data loss. Consider using VSCode as a git commit messag editor. [The official documentation](https://code.visualstudio.com/docs/sourcecontrol/overview#_vs-code-as-git-editor) states to use the `--wait` flag. Consider the following steps (reproducable): 1. Invoke `git commit`. VSCode opens with COMMIT_EDITMSG and Git says `hint: Waiting for your editor to close the file...` 2. Split the editor (horizontally or vertically). 3. Edit and save the commit message. 4. Close *one* of the editors. 5. Git's `code --wait` invocation completes and applies the commit. If our `git commit` terminal is behind our VSCode window, we have no idea that this happened. 6. We still have COMMIT_EDITMSG open and can edit it, oblivious to the fact that we have already committed. Therefore: VSCode interprets editor closure, which naturally happens when rearranging the editor layout, as editing-completion. This is just an example with git, where of course the commit message can be amended. However, other tools invoking VSCode with `--wait` can and do cause much more severe problems. Interestingly, the documentation linked above describes different behavior, when it says (emphasis mine) > When you launch VS Code from the command line, you can pass the `--wait…