Changes to the JSON schema

[larsgw]

I'm not sure if this repository accepts external issues but here I go:

• The schema URL is not documented, as far as I can tell. I see the editor uses https://raw.githubusercontent.com/WouterKoch/Clavis/main/Schema/Clavis.json, is that official?
• Field descriptions and examples of localizedString still refer to arrays while those seem to be objects now.
• The patterns of created and lastModified require the date to be in the 21st century. I guess this is fine if those dates signify when the Clavis file was created, not when the key itself was made (e.g. DELTA keys from before 2000).

I can make a PR for the latter two if you want.