Plugin to enforce strings immutability

Build a plugin to enforce localization strings immutability in various scenarios.

For example:
- Prevent the PR from merging if the values-*/strings.xml / Localizable.strings files were changed manually by a developer
- Warn developers when modifying existing keys in PRs
- Only allow (or disallow) string file modifications during code freeze

Start by reviewing the existing Danger plugin conventions in this repository, then map the requested behavior to values-*/strings.xml and Localizable.strings changes. Done means the plugin defines and documents how to block manual edits, warn about existing-key changes, and allow or disallow string-file changes during code freeze.

Written by the indexing model from the issue text.