(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

app/plugin/RenderOptions.ts

@@ -1,5 +1,7 @@
 /**
  * Where to append the content that a plugin renders.
+ *
+ * @internal
  */
 export type RenderTarget = "fullscreen" | number;