Add explicit "# key" directive to legacy snippet collection before deprecating filenames-as-triggers

snippets/snippet-mode/vars

@@ -1,11 +1,13 @@
 # -*- mode: snippet -*-
 # name : Snippet header
 # contributor : joaotavora
+# key: vars
 # --
 # name : $1${2:
 # key : ${3:trigger-key}}${4:
 # keybinding : ${5:keybinding}}${6:
 # expand-env : (${7:})}
 # contributor : $6
+# key: vars
 # --
 $0