Add Bortle-scale visual appearance data

[aaronlicle10]

Is your feature request related to a problem? Please describe.
Binocular-view rendering (10x) does not accurately account for light pollution, and object visibility during twilight varies in ways the software doesn't currently reflect.
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Binocular Views of Deep-Sky Objects from Bortle 8 to 5 (1).docx

[github-actions]

Hello @aaronlicle10!

Thank you for suggesting this enhancement.