Fix: `downlit_html_*()` keeps all pre classes

Close #149, close #156.

downlit_html() now keeps all pre classes, which allows for customization of code blocks in pkgdown using chunk options class.source and attr.source.

Old behavior (.my-class is not kept):

html <- xml2::read_xml("
  <div class='sourceCode'>
    <pre class='sourceCode r my-class'>
      <code class='sourceCode r'>
        <span>Hello</span>
      </code>
    </pre>
  </div>"
)
downlit::downlit_html_node(html)
html
#> {xml_document}
#> <div class="sourceCode">
#> [1] <pre class="downlit sourceCode r">\n<code class="sourceCode R"><span><spa ...

^{Created on 2022-08-05 by the reprex package (v2.0.1)}

New behavior (.my-class is kept):

html <- xml2::read_xml("
  <div class='sourceCode'>
    <pre class='sourceCode r my-class'>
      <code class='sourceCode r'>
        <span>Hello</span>
      </code>
    </pre>
  </div>"
)
downlit::downlit_html_node(html)
html
#> {xml_document}
#> <div class="sourceCode">
#> [1] <pre class="downlit sourceCode r my-class">\n<code class="sourceCode R">< ...

^{Created on 2022-08-05 by the reprex package (v2.0.1)}

@cderv You said that a fix for this should also fix https://github.com/r-lib/downlit/issues/133 but I didn't look at https://github.com/r-lib/downlit/issues/133 because of time and because it is related to several other issues.

Happy to modify the code and to bump version and NEWS if required.

Actually, this PR doesn't work because it applies all pre classes to all codeblocks, which is not the expected behavior:

html <- xml2::read_xml("
<div>
  <div class='sourceCode'>
    <pre class='sourceCode r my-class'>
      <code class='sourceCode r'>
        <span>Hello</span>
      </code>
    </pre>
  </div>
  <div class='sourceCode'>
    <pre class='sourceCode r another-class'>
      <code class='sourceCode r'>
        <span>Hello</span>
      </code>
    </pre>
  </div>
</div>"
)
downlit::downlit_html_node(html)
html
#> {xml_document}
#> <div>
#> [1] <div class="sourceCode">\n  <pre class="downlit sourceCode r my-class dow ...
#> [2] <div class="sourceCode">\n  <pre class="downlit sourceCode r my-class dow ...

^{Created on 2022-08-08 by the reprex package (v2.0.1)}

Therefore, I close it and will make a new one if I find a solution.

r-lib / downlit

Fix: `downlit_html_*()` keeps all pre classes #155