Configure JSON schema for `.cdb.json`

[AFCMS]

Fix https://github.com/GreenXenith/minetest-tools/issues/17

• [x] Configure JSON validation to the schema URL
• [x] Add a local fallback file (will need to be updated from time to time)

How to test?

• Open a new .cdb.json file
• Press Ctrl + Space in the brackets, check if Intellisense activates for possible fields
• Add a license, check if autocompletion works
• Check if hovering a license display "FOSS" if license is FOSS, "NON-FOSS" overwise
• Add some tags, check if autocompletion works
• Check if hovering tags display their description
• Test invalid definition, check if there are warning
• Disconnect from network
• Restart VSCode
• Check autocompletion still works (using the fallback file)