feat: add spec for login item revamp

[codebytere]

Initial spec for a revamp of app.{set|get}LoginItemSettings(settings).

[bitdisaster]

This change should support backwards compatibility/migration considering falling scenarios:
When launch on login was enabled with old version:

1. New version should report the setting via old API.
2. Disabling in new version should disable via old API
3. Enabling again should disable in the old API and enable in new API

[codebytere]

@bitdisaster oh good points - I can update the document to reflect what those cases! Does something like this:

If an app was set as a login item with the previous API, disabling the app as a login item will disable it with the old API. Enabling it again will use the new API. Additionally, it will continue to report login item status via old API until the app has been updated to use the new API.

sufficiently communicate your points in the document?

[erickzhao]

I don't think we need this at this stage, but having a migration guide for users of the removed APIs would be good.

Maybe it could be something as simple as a guidance on telling them to implement "open as hidden" as an app-level setting.

[codebytere]

electron/electron#37244 has now been merged.