`Shellwords.escape` escapes unquoted spaces with a backslash, but
Windows does not treat backslash as an escape character.  Escaping is
also a problem when paths are expressed in shortened 8.3 format (e.g.
`C:\Users\RubyOn~1\AppData\Local\Temp\...`) because a backslash will be
erroneously added before the `~`.

We can avoid the need to escape by using `system(command_name, *args)`
instead of `system(command_line)`, but we must still support
`ENV["EDITOR"]` values that embed command line arguments, such as
`subl -w`.

This commit changes to `system(command_name, *args)`, but uses
`Shellwords.split` to extract any embedded arguments from
`ENV["EDITOR"]`.  This requires that Windows users put quotes around the
entire path of their editor if it contains spaces, such as:

```
SET EDITOR="C:\Program Files\Microsoft VS Code\Code.exe" -w
```

In other words, the following are **not** supported on Windows:

```
SET "EDITOR=C:\Program Files\Microsoft VS Code\Code.exe"
SET EDITOR=C:\Program Files\Microsoft VS Code\Code.exe
SET EDITOR=C:\"Program Files"\"Microsoft VS Code"\Code.exe -w
SET EDITOR=C:\Program^ Files\Microsoft^ VS^ Code\Code.exe -w
SET EDITOR=C:\Program` Files\Microsoft` VS` Code\Code.exe -w
```

Fixes #41617 (again).
Closes #44890.