(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/dispose.d.ts

@@ -0,0 +1,16 @@
+// TODO: add remaining Disposable methode
+export abstract class Disposable {
+  public static create<T extends new (...args: any[]) => any>(
+    this: T, ...args: ConstructorParameters<T>): InstanceType<T>;
+
+  constructor(...args: any[]);
+  public dispose(): void;
+  public isDisposed(): boolean;
+  public autoDispose<T>(obj: T): T;
+  public autoDisposeCallback(callback: () => void): void;
+  public disposeRelease<T>(obj: T): T;
+  public disposeDiscard(obj: any): void;
+  public makeDisposable(obj: any): void;
+}
+
+export function emptyNode(node: Node): void;