(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/RenderOptions.ts

@@ -0,0 +1,11 @@
+/**
+ * Where to append the content that a plugin renders.
+ */
+export type RenderTarget = "fullscreen" | number;
+
+/**
+ * Options for the `grist.render` function.
+ */
+export interface RenderOptions {
+  height?: string;
+}