ts: Convert `compose_textarea.js` to TypeScript.

Added type definitions for `jquery-caret-plugin` third party package and
converted `compose_textarea.js` to TypeScript.

tools/test-js-with-node

@@ -67,7 +67,7 @@ EXEMPT_FILES = make_set(
         "web/src/compose_fade.js",
         "web/src/compose_recipient.js",
         "web/src/compose_state.js",
-        "web/src/compose_textarea.js",
+        "web/src/compose_textarea.ts",
         "web/src/compose_ui.js",
         "web/src/compose_validate.js",
         "web/src/composebox_typeahead.js",

web/src/compose_textarea.ts

@@ -4,7 +4,7 @@ import $ from "jquery";
 // shift-tab back in (see hotkey.js).
 let saved_compose_cursor = 0;
 
-function set_compose_textarea_handlers() {
+function set_compose_textarea_handlers(): void {
     $("#compose-textarea").on("blur", function () {
         saved_compose_cursor = $(this).caret();
     });
@@ -16,10 +16,10 @@ function set_compose_textarea_handlers() {
     });
 }
 
-export function restore_compose_cursor() {
+export function restore_compose_cursor(): void {
     $("#compose-textarea").trigger("focus").caret(saved_compose_cursor);
 }
 
-export function initialize() {
+export function initialize(): void {
     set_compose_textarea_handlers();
 }

web/src/global.d.ts

@@ -5,10 +5,26 @@
 
 declare let zulip_test: any; // eslint-disable-line @typescript-eslint/no-explicit-any
 
+type JQueryCaretRange = {
+    start: number;
+    end: number;
+    length: number;
+    text: string;
+};
+
 // eslint-disable-next-line @typescript-eslint/consistent-type-definitions
 interface JQuery {
     expectOne(): JQuery;
     tab(action?: string): this; // From web/third/bootstrap
+
+    // Types for jquery-caret-plugin
+    caret(): number;
+    caret(arg: number | string): this;
+    range(): JQueryCaretRange;
+    range(start: number, end?: number): this;
+    range(text: string): this;
+    selectAll(): this;
+    deselectAll(): this;
 }
 
 declare const ZULIP_VERSION: string;