Update htmlhint.json

[coliff]

Add additional rules.

[github-actions]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.

[madskristensen]

Thanks

[coliff]

thanks for the quick review Mads! I have one final change to make on this file but I don't know the exact way to specify it... the rules for attr-lowercase and attr-value-no-duplication should allow a boolean value OR an array. How can we set that?

I asked ChatGPT and it suggested this:

 "attr-lowercase": {
      "description": "Attribute name must be lowercase.",
      "anyOf": [
        { "type": "boolean" },
        { "type": "array" }
      ],
      "default": false
    },

Is that right?