Add display alias customisation point

This will allow environments such as P2P to tweak the preferred display alias if
needed.

src/customisations/Alias.ts

@@ -0,0 +1,31 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+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.
+*/
+
+function getDisplayAliasForAliasSet(canonicalAlias: string, altAliases: string[]): string {
+    // E.g. prefer one of the aliases over another
+    return null;
+}
+
+// This interface summarises all available customisation points and also marks
+// them all as optional. This allows customisers to only define and export the
+// customisations they need while still maintaining type safety.
+export interface IAliasCustomisations {
+    getDisplayAliasForAliasSet?: typeof getDisplayAliasForAliasSet;
+}
+
+// A real customisation module will define and export one or more of the
+// customisation points that make up `IAliasCustomisations`.
+export default {} as IAliasCustomisations;