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/range.js

@@ -0,0 +1,53 @@
+/*
+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 Range {
+    constructor(model, startPosition, endPosition = startPosition) {
+        this._model = model;
+        this._start = startPosition;
+        this._end = endPosition;
+    }
+
+    moveStart(delta) {
+        this._start = this._start.forwardsWhile(this._model, () => {
+            delta -= 1;
+            return delta >= 0;
+        });
+    }
+
+    expandBackwardsWhile(predicate) {
+        this._start = this._start.backwardsWhile(this._model, predicate);
+    }
+
+    get text() {
+        let text = "";
+        this._start.iteratePartsBetween(this._end, this._model, (part, startIdx, endIdx) => {
+            const t = part.text.substring(startIdx, endIdx);
+            text = text + t;
+        });
+        return text;
+    }
+
+    replace(parts) {
+        const newLength = parts.reduce((sum, part) => sum + part.text.length, 0);
+        let oldLength = 0;
+        this._start.iteratePartsBetween(this._end, this._model, (part, startIdx, endIdx) => {
+            oldLength += endIdx - startIdx;
+        });
+        this._model.replaceRange(this._start, this._end, parts);
+        return newLength - oldLength;
+    }
+}