Fix pasting HTML with newlines

[alecgibson]

At the moment, if you paste the following HTML, the newline between the
<span> elements is incorrectly discarded.

<span>foo</span>
<span>bar</span>

This will currently insert foobar into Quill, when we'd expect
foo bar, since newlines should treated as spaces between inline
elements (such as <span>).

This change updates the matchText() clipboard matcher to check if the
text node is between inline elements or not before early-returning.

[alecgibson]

A possible alternative solution could be:

function convertNewlinesToSpaces(root) {
  const walker = document.createTreeWalker(root, NodeFilter.SHOW_TEXT);
  let node = walker.nextNode();
  while (node) {
    if (!isPre(node)) node.textContent = node.textContent.replace(/\n/g, ' ');
    node = walker.nextNode();
  }
}

But that feels potentially a bit "aggressive"

[luin]

Cherry-picked to #3983