This commit adds a new optional parameter to the `newResource` function
of the `Mocks` interface for TypeScript. This can be useful when writing
tests which assert differing behavior between Custom and Component
resources.
Although the new parameter will always be set, the paramteter is marked
as optional in order to maintain backwards compatibility with existing
implementations of `Mocks`.
The tests are updated to verify that `custom` is set appropriately.
Co-authored-by: Luke Hoban <luke@pulumi.com>
