Add schema for Threagile models

[martingalloar]

Add schema for Threagile models

References:

• https://threagile.io/
• https://github.com/Threagile/threagile/blob/master/docs/model.md
• https://github.com/Threagile/threagile/blob/master/support/schema.json

[github-actions]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.

[hyperupcall]

Thanks