feat(jumplist): allow opting out of removing unloaded buffers (#29347)

Problem:  Cannot opt out of removing unloaded buffers from the jumplist.
Solution: Only enable that with "unload" flag in 'jumpoptions'.

test/old/testdir/setup.vim

@@ -13,6 +13,7 @@ if exists('s:did_load')
   set laststatus=1
   set listchars=eol:$
   set joinspaces
+  set jumpoptions=
   set mousemodel=extend
   set nohidden nosmarttab noautoindent noautoread noruler noshowcmd
   set nohlsearch noincsearch