river: Add shortcut to open papis library items

Added 'Mod+Shift+L' as shortcut to open the associated file of a
papis library item. May make more sense in a dedicated river
mode but for now is good enough to get an overview of papis items
and open them if needed. Could also probably use an equivalent
of 'open edit file' and 'open note if exists' but again -
let's see how much I use it first.

Takes a second to open with my library size.

desktop/.config/river/init

@@ -69,6 +69,9 @@ riverctl map normal $mod+Shift E spawn "bemoji -nt"
 # Open translation helper
 riverctl map normal $mod+Shift T spawn "bemenu-translate"
 
+# Open item from library
+riverctl map normal $mod+Shift L spawn "papis -s picktool dmenu open"
+
 # Desktop theming
 # shellcheck disable=SC2016
 riverctl map normal $mod+Shift S spawn 'flavourchoose'