Use YAML Extension for ASL and SSM YAML schemas

[whazor]

Problem

At the moment AWS Toolkit provides two built-in language server libraries for SSM and for ASL. Where both are a wrapper around yaml-language-server, which is the same library as the YAML extension is using. For SAM and CFN this is already done, so it makes sense to follow the same pattern for SSM and ASL as well.

[justinmk3]

ASL language server uses amazon-states-language-service to get validations/completions etc. for ASL. ASL requires contextual information that cannot be achieved with JSON Schema. For example, the "Next" property gives completions/validations for the next state name based on the states that the user already created. yaml-language-server package is being used in amazon-states-language-service to transform JSON validations/completions to YAML validations/completions.

[justinmk3]

WIP here:

• https://github.com/aws/aws-toolkit-common/pull/435
• https://github.com/aws/aws-toolkit-common/pull/433
• https://github.com/aws/aws-toolkit-common/pull/434