Propertize all shr fragment IDs as shr-target-id

Propertize all shr fragment IDs as shr-target-id

* lisp/net/shr.el (shr-target-id): Add docstring.
(shr-descend, shr-tag-a): Display dummy anchor characters as the
empty string.  Give all relevant 'id' or 'name' fragment identifier
attributes the shr-target-id text property.  This ensures that
cached content, such as tables, retains the property across
renders.  (Bug#40532)

* lisp/net/eww.el: (eww-display-html): Adapt shr-target-id property
search accordingly.