(core) move client code to core

Summary:
This moves all client code to core, and makes minimal fix-ups to
get grist and grist-core to compile correctly.  The client works
in core, but I'm leaving clean-up around the build and bundles to
follow-up.

Test Plan: existing tests pass; server-dev bundle looks sane

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2627

app/client/lib/testState.ts

@@ -0,0 +1,11 @@
+import {get as getBrowserGlobals} from 'app/client/lib/browserGlobals';
+import {TestState} from 'app/common/TestState';
+
+const G = getBrowserGlobals('window');
+
+export function setTestState(state: Partial<TestState>) {
+  if (!('testGrist' in G.window)) {
+    G.window.testGrist = {};
+  }
+  Object.assign(G.window.testGrist, state);
+}