(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/plugin/CustomSectionAPI.ts

@@ -0,0 +1,10 @@
+/**
+ * API definitions for CustomSection plugins.
+ */
+
+
+import {RenderTarget} from './RenderOptions';
+
+export interface CustomSectionAPI {
+  createSection(inlineTarget: RenderTarget): Promise<void>;
+}