ts: Convert compose_textarea.js to TypeScript.

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

web/src/compose_textarea.ts

@@ -0,0 +1,25 @@
+import $ from "jquery";
+
+// Save the compose content cursor position and restore when we
+// shift-tab back in (see hotkey.js).
+let saved_compose_cursor = 0;
+
+function set_compose_textarea_handlers(): void {
+    $("#compose-textarea").on("blur", function () {
+        saved_compose_cursor = $(this).caret();
+    });
+
+    // on the end of the modified-message fade in, remove the fade-in-message class.
+    const animationEnd = "webkitAnimationEnd oanimationend msAnimationEnd animationend";
+    $("body").on(animationEnd, ".fade-in-message", function () {
+        $(this).removeClass("fade-in-message");
+    });
+}
+
+export function restore_compose_cursor(): void {
+    $("#compose-textarea").trigger("focus").caret(saved_compose_cursor);
+}
+
+export function initialize(): void {
+    set_compose_textarea_handlers();
+}