Add support for Windows in secrets edit command

[aryanjassal]

Specification

The secrets edit command has never had a Windows implementation. As such, this command is currently defunct for Windows. This is a major usability issue and this must be addressed before multiplatform tests can be added onto the CI.

Windows - VISUAL, EDITOR, get the .txt file association first, then default to notepad.exe and wait for it to finish. Windows never had a default terminal text editor.

If the defaults cannot be found, you need to error out, indicating that no default editor could be found, and users can instead choose to set VISUAL, EDITOR or pass in the "editor" program with --ed or something.

Furthermore it is important that the temporary being created can only be edited by the same user as the user who ran polykey secrets edit, otherwise it can be intercepted. The file should have a limited umask, like 700 equivalent for Windows.

Additional context

• Relevant discussion about secrets edit command in Polykey-CLI#277

Tasks

1. Implement secrets edit for Windows
2. Make sure all the secrets edit tests are passing on Windows

[linear]

ENG-482 Add support for Windows in `secrets edit` command