(core) Forms feature

Summary:
A new widget type Forms. For now hidden behind GRIST_EXPERIMENTAL_PLUGINS().
This diff contains all the core moving parts as a serves as a base to extend this functionality
further.

Test Plan: New test added

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D4130

app/client/lib/domUtils.ts

@@ -1,9 +1,16 @@
+import {useBindable} from 'app/common/gutil';
 import {BindableValue, dom} from 'grainjs';
 
 /**
  * Version of makeTestId that can be appended conditionally.
- * TODO: update grainjs typings, as this is already supported there.
  */
 export function makeTestId(prefix: string) {
-  return (id: string, obs?: BindableValue<boolean>) => dom.cls(prefix + id, obs ?? true);
+  return (id: BindableValue<string>, obs?: BindableValue<boolean>) => {
+    return dom.cls(use => {
+      if (obs !== undefined && !useBindable(use, obs)) {
+        return '';
+      }
+      return `${useBindable(use, prefix)}${useBindable(use, id)}`;
+    });
+  };
 }