eng: add support for snapshot tests (#190444)

* eng: add support for snapshot tests

This adds Jest-like support for snapshot testing.
Developers can do something like:

```js
await assertSnapshot(myComplexObject)
```

The first time this is run, the snapshot expectation file is written
to a `__snapshots__` directory beside the test file. Subsequent runs
will compare the object to the snapshot, and fail if it doesn't match.

You can see an example of this in the test for snapshots themselves!

After a successful run, any unused snapshots are cleaned up. On a failed
run, a gitignored `.actual` snapshot file is created beside the
snapshot for easy processing and inspection.

Shortly I will do some integration with the selfhost test extension to
allow developers to easily update snapshots from the vscode UI.

For #189680

cc @ulugbekna @hediet

* fix async stacktraces getting clobbered

* random fixes

* comment out leak detector, for now

* add option to snapshot file extension