bug#50500: Add ways to copy id bookmark link in eww

[Juri Linkov]

>> Well, people put ids on all kinds of elements, but they're not generally
>> meant to be used for navigation.  (I mean, sometimes they are, but
>> statistically speaking, they're not.)
>>
>> So I'm not sure that `w' in general should be doing this in eww.
>> Anybody else got an opinion?
>
> Makes sense.  How about adding it as a customization variable, so that one
> can customize whether `w` also includes the id bookmark part and by default
> it does not.  It is often useful to save more precise positions in web
> page documents.

The problem is that "id" from such HTML:

  <h5 id="section13">13. Use with the GNU Affero General Public License.</h5>

is not rendered on UI, so the user doesn't know that here is some "id"
available for bookmarking.  But, for example, github.com for the sections
of the README.org file with ids displays visual indication that looks like
a chain icon on the left.  If eww will support some visual indication about
ids used on the page like github does, then it will make sense also to
include the displayed id to the copied link.