Generate frozen_string_literal comments in appraisals automatically

[mvz]

Summary

This adds customization to Appraisals to make it automatically add #frozen_string_literal: true to the generated gemfiles.

Motivation and Context

This makes it easier to update or create appraisals without having to autocorrect the result after.

How Has This Been Tested?

I ran appraisal generate and confirmed that there was no change to the gemfiles.

Types of changes

• Internal change (refactoring, test improvements, developer experience or update of dependencies)

Checklist:

N/A