todo_widget: Allow tasks to be added through /todo command.

Uptil now, users could add tasks to a todo widget only after creating
it through the `/todo` command in the compose box.

Users can now add an initial list of tasks using the `/todo` command,
with each task on a new line in the compose box, where the 1st `:`
would separate a task from its (optional) description. Example:
`/todo\nTask1:description1\nTask2 without description`.

Fixes part of #20213.

web/src/submessage.ts

@@ -32,6 +32,7 @@ const poll_widget_extra_data_schema = z
 export const todo_widget_extra_data_schema = z
     .object({
         task_list_title: z.string().optional(),
+        tasks: z.array(z.object({task: z.string(), desc: z.string()})).optional(),
     })
     .nullable();