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

snippets/rst-mode/chap

@@ -1,4 +1,5 @@
 #name : Chapter title
+# key: chap
 # --
 ${1:Chapter}
 ${1:$(make-string (string-width text) ?\=)}

snippets/rst-mode/sec

@@ -1,4 +1,5 @@
 #name : Section title
+# key: sec
 # --
 ${1:Section}
 ${1:$(make-string (string-width text) ?\-)}

snippets/rst-mode/tit

@@ -1,4 +1,5 @@
 #name : Document title
+# key: tit
 # --
 ${1:$(make-string (string-width text) ?\=)}
 ${1:Title}