Skipping URLs in verbatim elements didn't take nested
elements into consideration, which were not verbatim.
For instance, the following HTML snippet would yield
`https://example.com` in non-verbatim mode, even if
it is nested inside a verbatim `<pre>` element:
```html
<pre><a href="https://example.com">link</a></pre>
```
This commit fixes the behavior for both `html5gum` and
`html5ever`.
Note that nested verbatim elements of the same kind
still are not handled correctly.
For instance, the following HTML snippet would still yield
`https://example.com`:
```html
<pre>
<pre></pre>
<a href="https://example.com">link</a>
</pre>
```
The reason is that we currently only keep track of a single
verbatim element and not a stack of elements, which we
would need to unwind and resolve the situation.
Fixes https://github.com/lycheeverse/lychee/issues/986.
Previously remote URLs were incorrectly detected because the
string representation of a path is different than the path itself,
causing the `http` prefix match to be insufficient.
This resulted in unexpected side-effects, such as the
incorrect detection of verbatim mode for remote URLs.
The check now got improved and unit tests were added to avoid
future breakage. On top of that, missing verbatim elements were added
