(core) configure typedoc for generating plugin api documentation

Summary:
This annotates the plugin api sufficiently to generate some documentation
for it. See https://github.com/gristlabs/grist-help/pull/139

Contains some small code tweaks for things that caused typedoc some
trouble.

Test Plan: manual inspection of output

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3342

test/server/gristClient.ts

@@ -70,7 +70,7 @@ export class GristClient {
       if (this._pending.length) {
         return this._pending.shift();
       }
-      await new Promise(resolve => this._consumer = resolve);
+      await new Promise<void>(resolve => this._consumer = resolve);
     }
   }