(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/nbrowser/testServer.ts

@@ -90,7 +90,7 @@ export class TestServerMerged implements IMochaServer {
     // logging. Server code uses a global logger, so it's hard to separate out (especially so if
     // we ever run different servers for different tests).
     const serverLog = process.env.VERBOSE ? 'inherit' : nodeLogFd;
-    const env = {
+    const env: Record<string, string> = {
       TYPEORM_DATABASE: this._getDatabaseFile(),
       TEST_CLEAN_DATABASE: reset ? 'true' : '',
       GRIST_DATA_DIR: this.testDocDir,

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);
     }
   }