Add schema support for styles.filter.duotone

[georgeh]

What?

Add styles.filter.duotone to theme.json Schema

Closes #40398

Why?

This ensures that themes using the Duotone filter property will be able to validate correctly.

Testing Instructions

1. Create a theme.json with a duotone filter, ex:

{
  "$schema": "https://schemas.wp.org/trunk/theme.json",
  "version": 2,
  "styles": {
    "filter": {
      "duotone": "value"
    }
  }
}

2. Use a JSON schema validator to confirm it validates with no errors

Screenshots or screencast

N/A

[ajlende]

Thanks! Just one suggestion and one thing to think about.

[ajlende]

Just need the update to the .md reference and this looks good to go

[Mamaduka]

Hi, @georgeh

I think failing static analysis check is related to the changes.

There are local uncommitted changes after one or both of 'npm install' or 'npm run docs:build'!

[Mamaduka]

Thanks, @georgeh 🙇