Lexical declarations as JSX code children

[edemaine]

One more idea from #561: This PR allows for lexical declarations as JSX code children (as braced expressions, > expressions, or automatic code children). In particular, this example now works:

<div>
  user := getUser()
  if user?
    {name} := user
    <h1> `Welcome ${name}!`
  <h2>Posts</h2>
  for post of posts
    <div .post> post.jsx()

Somewhat unusual for Civet is that the declaration is not treated as an expression, as that would cause it to render in the JSX, which seems rarely desired. Instead we automatically add ,void 0 at the end to prevent rendering and just do the assignment. The declaration itself gets lifted (though there's no way to preserve const semantics here).