Support return types for yaml.customTags

[shin19991207]

What does this PR do?

This PR adds optional return types to the yaml.customTags setting.

Previously, a custom tag could only describe the YAML node type the tag is written on. For example, "!Seq-example sequence" tells the parser that !Seq-example would map to a sequence custom tag, but schema validation still treats the tagged value as a sequence.

This PR adds a new form "!Tag nodeType:returnType", where nodeType specifies the YAML node type that the tag is written on, and returnType specifies the schema type that the tagged value evaluates to.

yaml.customTags now supports three forms:

!Tag
!Tag nodeType
!Tag nodeType:returnType

Existing configurations remain supported, so this is backward compatible. Behavior only changes when a user explicitly adds :returnType.

What issues does this PR fix or reference?

Fixes redhat-developer/vscode-yaml#867

Note that yaml.customTags only applies to YAML short-form tags. It does not apply to CloudFormation long form such as Fn::.... Long form values are normal YAML mappings, not YAML tags.

Is it tested? How?

• New automated tests
• Manual testing:

1. Add the following yaml.customTags configuration in workspace setting (a list of all intrinsic functions in short form with return types added, based on the AWS CloudFormation intrinsic function docs):

{
  "yaml.customTags": [
    "!Base64 scalar:string",
    "!Cidr sequence:array",
    "!And sequence:boolean",
    "!Equals sequence:boolean",
    "!If sequence",
    "!Not sequence:boolean",
    "!Or sequence:boolean",
    "!Condition scalar:boolean",
    "!FindInMap sequence:string",
    "!GetAtt scalar:string",
    "!GetAtt sequence:string",
    "!GetAZs scalar:array",
    "!ImportValue scalar:string",
    "!Join sequence:string",
    "!Select sequence:string",
    "!Split sequence:array",
    "!Sub scalar:string",
    "!Sub sequence:string",
    "!Transform mapping",
    "!Ref scalar:string"
  ]
}

2. Create a file named template.yaml with the following content:

Resources:
  EC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      # example of short-form tag with block sequence input
      AvailabilityZone: !GetAtt
        - Ec2SecurityGroup
        - GroupId

      # example of short-form tag with flow sequence input
      ImageId: !FindInMap [AWSRegionArch2AMI, !Ref "AWS::Region", HVM64]

      # example of short-form tag with scalar input
      InstanceType: !Ref InstanceType

      SecurityGroupIds:
        # example of short-form tag inside an array item
        - !GetAtt
          - Ec2SecurityGroup
          - GroupId

3. Expect no errors.

[datho7561]

Looks good, works well and seems really helpful. Thanks, Morgan!