zform: Move zod schema to avoid dependency cycle.

We need to validate the events array in 'submessage.js' using
zod. One of the schema needed is 'zform_widget_extra_data_schema'
in zform. If we import it inside 'submessage', it creates a
dependency cycle ; zform.js=>transmit.js=>server_events.js=>server_events_dispatch.js.
server_events_dispatch.js=>submessage.js=>zform.js

Hence I have moved the schema inside 'submessage' and imported it
inside 'zform.js'.

web/src/submessage.js

@@ -1,8 +1,23 @@
+import z from "zod";
+
 import * as blueslip from "./blueslip";
 import * as channel from "./channel";
 import * as message_store from "./message_store";
 import * as widgetize from "./widgetize";
 
+export const zform_widget_extra_data_schema = z.object({
+    choices: z.array(
+        z.object({
+            type: z.string(),
+            long_name: z.string(),
+            reply: z.string(),
+            short_name: z.string(),
+        }),
+    ),
+    heading: z.string(),
+    type: z.literal("choices"),
+});
+
 export function get_message_events(message) {
     if (message.locally_echoed) {
         return undefined;

web/src/zform.js

@@ -1,24 +1,11 @@
 import $ from "jquery";
-import {z} from "zod";
 
 import render_widgets_zform_choices from "../templates/widgets/zform_choices.hbs";
 
 import * as blueslip from "./blueslip";
+import {zform_widget_extra_data_schema} from "./submessage";
 import * as transmit from "./transmit";
 
-const zform_widget_extra_data_schema = z.object({
-    choices: z.array(
-        z.object({
-            type: z.string(),
-            long_name: z.string(),
-            reply: z.string(),
-            short_name: z.string(),
-        }),
-    ),
-    heading: z.string(),
-    type: z.literal("choices"),
-});
-
 export function activate(opts) {
     const self = {};