Add JSON schema for book metadata

[solonovamax]

This is a recreation of #6, because I renamed the branch.

I was unsure where to put these, so I slapped them in the root directory.

I chose the root directory for consistency, as owo-ui.xsd is in the root directory for owo-lib.
If you would like me to place them elsewhere, just let me know and I can do that.

Also, I did not include icon_sprite in the schema, as even though the docs mention it, after looking at the code, it is not a valid property and will just be ignored.

The docs should also be appropriately updated to tell users to include

"$schema": "https://raw.githubusercontent.com/wisp-forest/lavender/1.20.3/lavender-book.json"

at the top of their book's json file,

"$schema": "https://raw.githubusercontent.com/wisp-forest/lavender/1.20.3/lavender-category.json"

in the json metadata for their categories, and

"$schema": "https://raw.githubusercontent.com/wisp-forest/lavender/1.20.3/lavender-entry.json"

in the json metadata for their entries.