Move tabindex=0 to the direct parent div for code blocks

[rianrietveld]

From the accessibility audit, issue #232

SC 2.2.2 keyboard: The code blocks are scrollable but cannot be focussed with the keyboard. That means keyboard only users can't read code that overflows the container. The code examples must be made focusable, that way scrolling with the arrow keys is possible. Firefox autmatically makes overflow boxes scrollable, so it's easy to miss the issue when Firefox is your default browser. This occurs on all pages with code blocks, for example https://wpaccessibility.org/docs/topics/content/alt-text/

Note Rian: There is a tabindex on the code blocks, but to make axe Devtools shut up it must be on the direct container <div class="highlight"> and not on the wrapper <div class="language-html highlighter-rouge" tabindex="0">.

<div class="language-html highlighter-rouge" tabindex="0">
  <div class="highlight">
    <pre class="highlight">
      <code>
        [...]

The work:
Move tabindex="0" to <div class="highlight">.