Add airline generator

[wonderer007]

Motivation / Background

We have been using Faker::Travel::Airport and needed Faker::Travel::Airline, so I created one and opened this PR despite knowing maintainers aren't encouraging new generators.

Checklist

Before submitting the PR, make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug, refactor something, or add a feature.
• Tests and Rubocop are passing before submitting your proposed changes.

If you're proposing a new generator or locale:

• Double-check the existing generators documentation to make sure the new generator you want to add doesn't already exist.
• You've reviewed and followed the Contributing guidelines.

[thdaraujo]

Thanks for your contribution, @wonderer007!

But we are not accepting new generators right now. Please take a look at our Contributing docs to see the types of contributions we are looking for!

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Similar to #1663