Add escaping where needed

[kienstra]

• [x] Fix composer phpcs issues
• [x] Add more escaping where needed
• [x] Bump version to 3.1.0 in preparation for a release

How to test

1. No build steps needed for this plugin, like npm run dev, etc…
2. Activate Twenty Twenty-One, or any non-FSE theme
3. Go to the Customizer
4. Add the 'Simple Social Icons' widget
5. Populate all of the fields. For testing, URLs can all simply be https://example.com.
6. Expected: The widget's markup has the edits that you made in the fields:

Documentation

No documentation required (I think)

Suggested changelog entry

• Add escaping to the widget's output

[kienstra]

Sorry, not planning any more commits before review.

[kienstra]

@dreamwhisper, thanks a lot for approving this!