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

snippets/c++-mode/beginend

@@ -1,3 +1,4 @@
 #name : v.begin(), v.end()
+# key: beginend
 # --
 ${1:v}.begin(), $1.end

snippets/c++-mode/class

@@ -1,4 +1,5 @@
 #name : class ... { ... }
+# key: class
 # --
 class ${1:Name}
 {

snippets/c++-mode/ns

@@ -1,3 +1,4 @@
 #name : namespace ...
+# key: ns
 # --
 namespace 

snippets/c++-mode/template

@@ -1,3 +1,4 @@
 #name : template <typename ...>
+# key: template
 # --
 template <typename ${T}>

snippets/c++-mode/using

@@ -1,4 +1,5 @@
 #name : using namespace ... 
+# key: using
 # --
 using namespace ${std};
 $0