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

snippets/snippet-mode/field

@@ -1,5 +1,6 @@
 # name : ${ ...  } field
 # contributor : joaotavora
 # key : $f
+# key: field
 # --
 \${${1:${2:n}:}$3${4:\$(${5:lisp-fn})}\}$0

snippets/snippet-mode/mirror

@@ -1,5 +1,6 @@
 # name : ${n:$(...)} mirror
 # key : $m
 # contributor : joaotavora
+# key: mirror
 # --
 \${${2:n}:${4:\$(${5:reflection-fn})}\}$0

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