feat: merge serializable jsx children

[marvinhagemeister]

This PR adds some logic to the new JSX transforms that allows it to merge serializable JSX children together. A node is deemed serializable when it is text, or is a serializable HTML node. This addresses the following scenarios where we'd generate more templates than necessary:

// input
<div>foo{" "}bar</div>

// before
const tpl = [
  "<div>foo",
  "",
  "bar</div>"
]
jsxssr(tpl, " ")

// after
const tpl = [
  "<div>foo bar</div>",
]
jsxssr(tpl)

This scenario happens often enough in our code bases that this optimization should be worth it.