Support serverless variables syntax ${}

[mnapoli]

This is a follow up from serverless/serverless#3184 (comment)

With a little changes, I managed to make this plugin support the ${} syntax of Serverless (instead of #{}). Here is an example:

functions:
    hello:
        handler: handler.hello
        environment:
            # We can reference `AWS::` variables
            REGION: ${AWS::Region}

            # We can use !Ref inline via the `ref:` prefix:
            BUCKET_NAME: ${ref:MyBucket}

            # We can use !GetAtt inline via the `getatt:` prefix:
            BUCKET_URL: 'https://${getatt:MyBucket.DomainName}'

            # We can use mix all that together in a string:
            REGIONAL_URL: 'https://${ref:MyBucket}.s3.${AWS::Region}.amazonaws.com'

            # We can also mix all of that with actual serverless variables (e.g. `${self:provider.stage}`)

resources:
    Resources:
        MyBucket:
            Type: AWS::S3::Bucket

I defined the following variables:

• ${AWS::xxx} is compiled to #{AWS::xxx}
• ${ref::xxx} is compiled to #{xxx}
• ${getatt::xxx.yyy} is compiled to #{xxx.yyy}

I am opening this PR first to get some feedback and see if we should proceed with this. I like the fact that it is backward compatible, yet opens the possibility to switch to an entirely new syntax.

WDYT?

[medikoo]

Closing, as all functionalities as provided by this plugin are now covered by the Framework (since v2.50.0)