Convert editor to TypeScript

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/editor/offset.ts

@@ -0,0 +1,30 @@
+/*
+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.
+*/
+
+import EditorModel from "./model";
+
+export default class DocumentOffset {
+    constructor(public offset: number, public readonly atNodeEnd: boolean) {
+    }
+
+    asPosition(model: EditorModel) {
+        return model.positionForOffset(this.offset, this.atNodeEnd);
+    }
+
+    add(delta: number, atNodeEnd = false) {
+        return new DocumentOffset(this.offset + delta, atNodeEnd);
+    }
+}