Adds captions in the viewer

ts/util/graphemeAwareSlice.ts

@@ -0,0 +1,34 @@
+// Copyright 2021-2022 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export function graphemeAwareSlice(
+  str: string,
+  length: number,
+  buffer = 100
+): {
+  hasReadMore: boolean;
+  text: string;
+} {
+  if (str.length <= length + buffer) {
+    return { text: str, hasReadMore: false };
+  }
+
+  let text: string | undefined;
+
+  for (const { index } of new Intl.Segmenter().segment(str)) {
+    if (!text && index >= length) {
+      text = str.slice(0, index);
+    }
+    if (text && index > length) {
+      return {
+        text,
+        hasReadMore: true,
+      };
+    }
+  }
+
+  return {
+    text: str,
+    hasReadMore: false,
+  };
+}