HTML and CSS services should collect custom data added by other extensions

[Princesseuh]

In VS Code, extensions can add custom data through the following code:

{
  "contributes": {
    "html": {
      "customData": ["./html.html-data.json"]
    },
    "css": {
      "customData": ["./css.css-data.json"]
    }
  }
}

At this time, this custom data isn't found by the HTML and CSS services, leading to downstream issues like this one: https://github.com/withastro/language-tools/issues/700