Add a migration script

stpkg

@@ -937,7 +937,7 @@ __migrate() {
     local pack_name="$1"
     [ "x$pack_name" = "x" ] && die "You must specify a pack for the 'migrate' command"
 
-    local FILE="${STPKG_INSTALL}web/packs/$pack_name.json"
+    local FILE="${STPKG_INSTALL}/web/packs/$pack_name.json"
     [ ! -r "$FILE" ] && die "Pack '$pack_name' is not available (check with 'pack list'). The corresponding file should be '$FILE'"
     echo "Migrate the sticker pack $(__green `jq '.title' < "$FILE"`):"