add support for selecting ranges in the editor model, and replacing them

this to support finding emoticons and replacing them with an emoji

src/editor/position.js

@@ -0,0 +1,38 @@
+/*
+Copyright 2019 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.
+*/
+
+export default class DocumentPosition {
+    constructor(index, offset) {
+        this._index = index;
+        this._offset = offset;
+    }
+
+    get index() {
+        return this._index;
+    }
+
+    get offset() {
+        return this._offset;
+    }
+
+    compare(otherPos) {
+        if (this._index === otherPos._index) {
+            return this._offset - otherPos._offset;
+        } else {
+            return this._index - otherPos._index;
+        }
+    }
+}