(core) Floating formula editor

Summary:
Adding a way to detach an editor. Initially only implemented for the formula editor, includes redesign for the AI part.
- Initially, the detached editor is tight with the formula assistant and both are behind GRIST_FORMULA_ASSISTANT flag, but this can be relaxed
later on, as the detached editor can be used on its own.

- Detached editor is only supported in regular fields and on the creator panel. It is not supported yet for conditional styles, due to preview limitations.
- Old code for the assistant was removed completely, as it was only a temporary solution, but the AI conversation part was copied to the new one.
- Prompting was not modified in this diff, it will be included in the follow-up with more test cases.

Test Plan: Added only new tests; existing tests should pass.

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Differential Revision: https://phab.getgrist.com/D3863

test/server/testUtils.ts

@@ -303,6 +303,10 @@ export class EnvironmentSnapshot {
       }
     }
   }
+
+  public get(key: string): string|undefined {
+    return this._oldEnv[key];
+  }
 }
 
 export async function getBuildFile(relativePath: string): Promise<string> {