update types, deprecate lowercase event handlers

[titoBouzout]

It has been noted that supporting both onclick and onClick makes components a bit more complicated to type and support, as a consumer could use any of both. Literally copying and pasting the issue

oh it's just the same old issue: If JSX.ButtonHTMLAttributes has both onclick and onClick, then when writing a component to wrap it, you either have to pick one or the other Omit<JSX.ButtonHTMLAttributes, "onClick"> or you have to support both. Consider

function PartyButton(props: JSX.ButtonHTMLAttributes<HTMLButtonElement>) {
  let button: HTMLButtonElement | undefined;
  return (
    <button
      {...props}
      ref={fwd(props, ref => (button = ref))}
      onclick={e => (
        triggerConfettiAnimation(button),
        typeof props.onclick === "function" ? props.onclick?.(e) : undefined
      )}
    />
  );
}

now when someone uses this <PartyButton onClick={() => console.log("clicked")}>Example</PartyButton> the onClick doesn't get called

From https://discord.com/channels/722131463138705510/817960620736380928/1313201415182745630 by @webstrand

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Kobalte and corvu both use camelCase events.

The eslint plugin also prefers onClick over onclick, and by default will autocorrect accordingly. (But there's an option to ignoreCase) [from @edemaine ]