Add schema validation workflow

[Hans5958]

This PR adds a schema validation workflow for addon manifests, based on the schema that I made on Hans5958/ScratchAddons-Schema, with some actions that I have modified so it works properly.

The workflow have been tested rigorously by myself and it seems that it is working properly.

This pull request partially resolves #229 as there will be future PRs for updating all addon manifest for the schema, which will be done after this PR.

Some notes:

• There is still a need to get the descriptions done on the schema. I have filled some of the description based on the wiki, but there still some field missing, indicated by the TODO comments.
• Tell me if you need to move the repositories from mine to this organization.
• I made this so it only works on the master branch of this repo only. Should I make it work on all forks and branches also? It's quite easy, don't worry.

[Hans5958]

@WorldLanguages please answer questions