web components: implement parsing logic for the custom elements manifest file format

[kristoff-it]

https://github.com/webcomponents/custom-elements-manifest

The parser should live in src/html and collect information about custom attributes and which categories the element inherits from its parent element (if any).

I'm not too familiar with the spec so I might be missing some important detail, but the goal is to collect the information in an efficient manner (so consider creating instances of html.Element for example) for the purpose of validation, autocompletion and showing descriptions.

Note that Zig has a json parser so in this case "parsing" is about organizing and processing data obtained by the json.

The integration work for this new parser is not contributor friendly so this is the perfect stopping point for a PR. See the parent Issue for more information.