Show the first new element for outerHTML swaps that start with whitespace

[cestercian]

Description

With hx-swap="outerHTML show:top" (or show:bottom), the swap target after an outerHTML swap is the first node of the new content. When the response starts with whitespace, that node is a text node, which has no scrollIntoView, so nothing scrolls.

This picks the first element of the new content instead, falling back to the first node (or the parent) like before when there isn't one. This is what was suggested in the issue.

Corresponding issue: Fixes #4114

Testing

Added two tests in test/tests/attributes/hx-swap.js for outerHTML show:top and show:bottom with a response that starts with whitespace. Both fail without the change. npm test passes locally in Chromium (1782 passed, 4 skipped).

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded