Improve type-safety of string-based props

[davesnx]

Many attributes are defined as string when in reality (the HTML spec) doesn't allow any string and it defines an enum. There's the case of attributes defined also as a polymorphic typeint or bool as well or more strange

The benefit from moving away from string-based API are:

• Safety, within a string there could be a typo without noticing it.
• Learnability. Users might be able to learn about HTML and their APIs.
• Correctness, generate valid HTML isn't crazy important but it's nice to have a feedback on compile time, also have a positive impact on on-page SEO.

There have been a few efforts on rescript-react to push that direction but didn't land yet (a few comments by @dodomorandi in rescript-lang/syntax#235) and I'm not sure if there's a positive intent to do so.

We have the possibility to do it

Other references:

• Polyvariant attributes: to bump or not to bump rescript-lang/rescript-react#10
• https://github.com/rescript-lang/rescript-react/pull/12/files
• WIP: React JSX PPX v4 rescript-lang/syntax#235

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The plan for this is to check TyXML and https://html.spec.whatwg.org/#global-attributes and manually ensure that most of our attributes make sense.

Doubts

• How to provide a way to override/discard the issue?
• How easy is to discover the attrs values?
• How we can expose Ppx_static_attributes on both the ppx and the runtime? (Maybe merging Jsx.Attributes and Ppx_static_attributes