Integrate the Applet runner into the toolkit. When the --app argument
points to a .yml or .yaml file, the target will be assumed to be a
JavaScript applet, and run through the applet interpreter.

Enhancements to JS applet interpreter:

- Allow using non-Stack constructs.
- Multiple Stacks in one applet file by supplying an array.
- Allow referencing packages directly from NPM.

Non-Stack constructs can now be used as applets. If a non-Stack applet
is selected, a Stack will be constructed around it. This makes it
possible to reuse Applet constructs in regular CDK applications (if they
meet the requirements).

Multiple applets can be define in one file by making the top-level
object in the YAML file a list. Each applet will get its own stack.

By starting an applet specifier with npm://, applet modules can
directly be referenced in NPM. You can include a version specifier
(@1.2.3) to reference specific versions.