feat: remove deprecated additionalFeatures

[nornagon]

Description of Change

This removes the deprecated additionalFeatures key from the new-window and
did-create-window events. It is retained in new-window as an argument but
will always be the empty array [], because that event uses positional
arguments.

The new behavior is that bare keys will end up as true in the options
argument.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Removed deprecated additionalFeatures property from new-window and did-create-window WebContents events.

[nornagon]

API review bot is broken. Merging as this has been approved.

[release-clerk]

Release Notes Persisted

Removed deprecated additionalFeatures property from new-window and did-create-window WebContents events.