Ensure support for intrinsics YAML shorthand

[mrgrain]

In Yaml, a user can write !GetAtt MyBucket.BucketName or !Ref MyBucket to refer to intrinsic functions.

Currently we don't have any tests or examples covering these. Need to make sure we support them everywhere.

Also need to check:

• Does it work with Yaml Schema Validation?
• Does it work with custom Intrinsic Functions? (Currently only CDK::GetProp) Should it?

[mrgrain]

Looks like there is some support for it already:

decdk/src/parser/private/cfn-yaml.ts

Line 5 in 615636f

const shortForms: yaml_types.Schema.CustomTag[] = [

[lejiati]

Yea it does. I tested with a sample template and it indeed passed test suite. But lacking support for parameter/condition/mapping limits what tests case we can have. So I'm thinking we can defer this task until more features were added.