bug#58118: 29.0.50; Follow #target links in eww without re-rendering page

[Visuwesh]

Severity: wishlist

eww-follow-link currently follows #target links in the same URL by
re-rendering the page.  Whilst this is fine for small HTML files, the
1.9M HTML file that I'm currently visiting takes a couple seconds to
render.
I see that eww-display-html follows #target links by a text property
search, why don't we do the same in eww-follow-link as well?  I have one
question though: what do we about non-existent #targets?  Chrome and
Firefox seem to not scroll when I add a non-existent #target to the
current URL and say RET, and it seems to specially handle #top to mean
to go to the top even if the HTML has no hits for "#top".  Should we
handle the special #top target?
By non-existent targets, I mean to say that

    (goto-char (point-min))
    (text-property-search-forward 'shr-target-id "invalid" #'member)

returns nil for a non-existent "#invalid" target.

Tentative patch attached,

eww-target.patch
Description: Text Data

In GNU Emacs 29.0.50 (build 31, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw scroll bars) of 2022-09-27 built on astatine
Repository revision: 51ec68b3182914c6c95d6ddcd88ae75239ec2904
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Debian GNU/Linux bookworm/sid

Configured using:
 'configure --with-x-toolkit=lucid --without-xaw3d'
Configured features:
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Load-path shadows:
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Memory information:
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