Fix order of GetTerms

Preserve the order from front matter, which would be behaviour when doing this manually (before GetTerms).

Fixes #7213

common/collections/order.go

@@ -0,0 +1,20 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package collections
+
+type Order interface {
+	// Ordinal is a zero-based ordinal that represents the order of an object
+	// in a collection.
+	Ordinal() int
+}