Make imenu index generation for PDFs more reliable

Make imenu index generation for PDFs more reliable

Do away with parsing the output of "mutool show FILE outline"
since the URI reported in its output may not include the page
number of the heading, and instead may contained "nameddest"
elements which cannot be resolved using "mutool".  Instead, use
a MuPDF JS script to generate the PDF outline allowing to
resolve such URIs.

* lisp/doc-view.el (doc-view--outline-rx): Remove as no longer
needed.
(doc-view--outline): Reflect that outline can be generated for
non-PDF files too.
(doc-view--mutool-pdf-outline-script): Add new variable to hold
the JS script used to generate the outline.
(doc-view--pdf-outline): Use the script.  (bug#73638)

(cherry picked from commit 44aa385b9afe2f2d500f62fd679314150e859d77)