feat_req(html): check whether a html attribute is boolean according to the spec

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Is your feature request related to a problem? Please describe.

I hope the pandora box wasn't opened with #5437, but I was hoping for some utilitary function that'd be able to tell if an html attribute for a given tagname is a boolean attribute or not.

Spec refs:

• https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#boolean-attribute
• https://html.spec.whatwg.org/#attributes-3

The second link is a table containing a list of tagname/attributes and allowed values which can be used as reference. It is complete enough to also implement other utilities function.

Why the focus on boolean attributes ?

• These can often be misused, because of how boolean attributes are handled in html: a falsy value is when it is omitted, while a truthy value is its presence (including empty value or its name itself)
  • e.g. disabled="false" is actually treated as disabled="disabled" by most browsers, in addition to being technically incorrect
• Such a function could be used for simple linting/formatting/sanitizing/testing tools, helping when generating html templates, etc.
• It makes the scope relatively small for now if std thinks this can be in-scope for the html package

As the spec is well-defined, it wouldn't require much maintenance, while once again abstracting the complexity of the html spec for users.

Describe the solution you'd like

Something along:

isBooleanAttribute("VIDEO", "playsinline") // true
isBooleanAttribute("DIV", "playsinline") // false

Describe alternatives you've considered

There's is-boolean-attribute but technically it's not compliant since it doesn't check the tagname