general: Add default nano editor for the edit alias

[NoahGorny]

Provide a default editor in case EDITOR and ALTERNATIVE_EDITOR are not defined

Motivation and Context

Closes #2123

How Has This Been Tested?

Locally, I can probably add a test of this

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[cornfeedhobo]

@NoahGorny I'm not sure how I feel about this PR. I feel like vi is the correct fall back since it's the standard editor for most distros. However, I'm probably bike shedding and don't think the impact is big enough to really care. just my $0.02