feat(help): hide filename of "gO" outline using conceal #27547

Help outlines, invoked by `gO`, displays the help section titles in the
location list window. This feature is implemented by setting the buffer
lines after opening the window, but this implementation breaks the
assumption that the quickfix window texts are consistently constructed
by the quickfix list items. I think we can use the conceal feature here.
Using conceal here improves interoperability between quickfix plugins,
and also simplifies the outline implementation.

Originally reported at https://github.com/itchyny/vim-qfedit/issues/12

runtime/syntax/qf.vim

@@ -15,6 +15,12 @@ syn match	qfSeparator	"|" nextgroup=qfLineNr contained
 syn match	qfLineNr	"[^|]*" contained contains=qfError
 syn match	qfError		"error" contained
 
+" Hide file name and line number for help outline (TOC).
+if has_key(w:, 'qf_toc') || get(w:, 'quickfix_title') =~# '\<TOC$'
+  setlocal conceallevel=3 concealcursor=nc
+  syn match	Ignore		"^[^|]*|[^|]*| " conceal
+endif
+
 " The default highlighting.
 hi def link qfFileName	Directory
 hi def link qfLineNr	LineNr