Keep track of match extents in occur-mode (bug#39121)

Keep track of match extents in occur-mode (bug#39121)

Use the `occur-target` text property to keep track of the extents of
all matches on each line instead of just the start of the first match.
Doing so allows us to highlight all matches when jumping to a matching
line instead of just the first one, and it works in a more principled
way.  It also removes compatibility problems that were introduced with
occur-highlight-regexp.

For compatibility with code that populate their own occur-mode
buffers, we still accept `occur-target` properties with a single
marker as value.

* lisp/replace.el (occur-highlight-regexp, occur-highlight-overlay):
Remove.
(occur-highlight-overlays): New.
(occur--targets-start): New.
* lisp/replace.el (occur-after-change-function):
(occur-mode-find-occurrence): Replace with...
(occur-mode--find-occurrences): ...this function that returns the
whole `occur-target` property value.
(occur-mode-goto-occurrence, occur-mode-goto-occurrence-other-window)
(occur-goto-locus-delete-o, occur-mode-display-occurrence)
(occur-engine): Adjust to new property format.
(occur--highlight-occurrence): Replace with...
(occur--highlight-occurrences): ...this function that takes
the `occur-target` property value as argument.
(occur-1): Don't use `occur-highlight-regexp`.
* test/lisp/replace-tests.el (occur-highlight-occurrence):
Adapt to new property format.