(core) add grist.onRecord and grist.onRecords event handlers

Summary:
This simplifies writing custom widgets that access selected
data.  To access the record at which the cursor is set, and
get any future changes to it as the cursor moves or data
changes, it suffices now to do:

```
grist.ready();
grist.onRecord(record => /* render */);
```

Similarly to access the set of selected records, and get any
changes, it suffices now to do:

```
grist.ready();
grist.onRecords(records => /* render */);
```

The `records` argument will be a list of objects, each of which
is a single record.  This is distinct from the column-based
representation favored in Grist up ontil now.  That remains
how methods like `fetchTable` or `fetchSelectedTable` represent
their results.  In the future, methods named like `fetchRecords`
or `fetchSelectedRecords` could be added that return lists.

Test Plan: extended tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2583

app/plugin/GristData.ts

@@ -0,0 +1,6 @@
+export type CellValue = number|string|boolean|null|[string, any?];
+
+export interface RowRecord {
+  id: number;
+  [colId: string]: CellValue;
+}