Plan for JSX V4.

[cristianoc]

With the introduction of more general structural typing https://forum.rescript-lang.org/t/ann-more-strict-checks-in-missed-labels/2117 most of the JSX ppx could be removed. In particular, one could just use functions from the language directly.

A plan would require

• Describe precisely how it works. (See https://github.com/rescript-lang/syntax/pull/517/files#diff-c97c31727a92fde7cad7a258699c4d6bf67cba2654d93992c758bf94df5f8411).
• Evaluate whether it is true that one can just "use the language" to define functions for components. Find out if there are issues and what these are.
• Figure out a solution to the expressivity problem of @obj records: JSX V4 WIP #517 (comment)
• Check that hover and jump to location in the editor work.
• Check that type error messages report the desired location.
• Explore JSX preserve mode (i.e. output jsx syntax in the generated code). See Explore JSX preserve mode (i.e. output jsx syntax in the generated code) #539.
• Explore React's own jsx transform (https://forum.rescript-lang.org/t/jsx-v4-next-rescript-react-0-10-x/3431/10)
• Plan a migration path. And what about dependencies.
• Engage with the community for feedback. This would be a major visible change. (See forum post)

[cristianoc]

++ @rickyvetter

[ryyppy]

Things to consider (that may or may not be solved by simplifying the ppx):

• Make sure it's easy to comply to the fast-refresh naming rules (capitalized functions, no leading $$) -> also see this forum post

(will probably edit this comment later to add more items)