Try to open external PDF reader to page number for "Show on Page"

PDF-handling code from `ZoteroPane.viewAttachment()` predates
`Zotero.OpenPDF` and didn't handle page numbers for external PDF
readers, so when called from "Show on Page" in a note (via
`ZoteroPane.viewPDF()`) it just launched the file itself in the
specified handler or via the OS.

For now, change to use `Zotero.OpenPDF.openToPage()` when there's a page
number. Later we should use `Zotero.OpenPDF` for everything, but that
code is more complicated than just using `launchFile()`, so we'll need
to do some testing (particularly on Linux) to see what's best.

(Note that "Show on Page" with an external reader doesn't make a ton of
sense, since you can't actually see the annotation, but opening the
right page is better than nothing.)

Fixes #2612

chrome/content/zotero/zoteroPane.js

@@ -4284,7 +4284,19 @@ var ZoteroPane = new function()
 					);
 					return;
 				}
+				// Try to open external reader to page number if specified
+				else {
+					let pageIndex = extraData?.location?.position?.pageIndex;
+					if (pageIndex !== undefined) {
+						await Zotero.OpenPDF.openToPage(
+							item,
+							parseInt(pageIndex) + 1
+						);
+						return;
+					}
+				}
 				// Custom PDF handler
+				// TODO: Remove this and unify with Zotero.OpenPDF
 				if (pdfHandler != 'system') {
 					try {
 						if (await OS.File.exists(pdfHandler)) {