(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/domAsync.ts

@@ -0,0 +1,25 @@
+import {reportError} from 'app/client/models/errors';
+import {DomContents, onDisposeElem, replaceContent} from 'grainjs';
+// grainjs annoyingly doesn't export browserGlobals tools, useful for testing in a simulated environment.
+import {G} from 'grainjs/dist/cjs/lib/browserGlobals';
+
+/**
+ * Insert DOM contents produced by a Promise. Until the Promise is fulfilled, nothing shows up.
+ * TODO: This would be a handy place to support options to show a loading spinner (perhaps
+ * showing up if the promise takes more than a bit to show).
+ */
+export function domAsync(promiseForDomContents: Promise<DomContents>, onError = reportError): DomContents {
+  const markerPre = G.document.createComment('a');
+  const markerPost = G.document.createComment('b');
+
+  // Function is added after the markers, to run once they have been attached to elem (the parent).
+  return [markerPre, markerPost, (elem: Node) => {
+    let disposed = false;
+    promiseForDomContents
+      .then((contents) => disposed || replaceContent(markerPre, markerPost, contents))
+      .catch(onError);
+
+    // If markerPost is disposed before the promise resolves, set insertContent to noop.
+    onDisposeElem(markerPost, () => { disposed = true; });
+  }];
+}