feat: narrowed type for categories in icon.schema.json

[jguddas]

What is the purpose of this pull request?

• [ ] New Icon
• [ ] Bug fix
• [ ] New Feature
• [ ] Documentation update
• [x] Other:

Description

Before the categories field in icons/*.json could be any string, now it can only be a valid category name.

Before Submitting

• [ ] I've read the Contribution Guidelines.
• [ ] I've checked if there was an existing PR that solves the same issue.