Closes #2706

This PR cuts down the JSX namespace to only include the actual JSX interfaces, i.e., Element, IntrinsicElements, IntrinsicAttributes, etc. We've been using it to hold most of our DOM-ish typings forever which is a) surprising for users looking for these types, if they can find them at all and b) probably not ideal given the magic handling of JSX by TS. Best to keep it pretty minimal and only load it up with the interfaces expected of it.

This will let users import types directly from Preact, like so:

// Before
import { JSX } from 'preact';

let MyButtonProps: JSX.ButtonHTMLAttributes & {
  //...
}

// After
import { ButtonHTMLAttributes } from 'preact';

let MyButtonProps: ButtonHTMLAttributes & {
  //...
}

Moving these out to a separate declaration file seemed easiest, as we need to re-export all of these types except for the JSXInternal namespace and I found that to be quite fiddly; I frequently broke IntrinsicElements when working on that and so just moved the unrelated types out. Seems less error-prone like this, and does somewhat nicely isolate the JSX-specific types. Might be easier to maintain.

Big ol' breaking change of course, but for the better I think. Should be pretty quick for consumers to fix up.