Add Phonelib area code

[bdewater]

This bit me when trying to use the area code with delete_prefix, which doesn't accept nil.

Type of Change

• [ ] Add RBI for a new gem
• [x] Modify RBI for an existing gem
• [ ] Other:

Changes

• Gem name: Phonelib
• Gem version: 0.9.1
• Gem source: https://github.com/daddyz/phonelib/blob/466a328ec14d25e95542dc7eb70a358e051620bb/lib/phonelib/phone_formatter.rb#L96-L108

[egiurleo]

Thanks @bdewater!