(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-ti.ts

@@ -0,0 +1,11 @@
+import * as t from "ts-interface-checker";
+// tslint:disable:object-literal-key-quotes
+
+export const CustomSectionAPI = t.iface([], {
+  "createSection": t.func("void", t.param("inlineTarget", "RenderTarget")),
+});
+
+const exportedTypeSuite: t.ITypeSuite = {
+  CustomSectionAPI,
+};
+export default exportedTypeSuite;