Use YAML Extension for ASL and SSM YAML schemas

## Problem
At the moment AWS Toolkit provides two built-in language server libraries for [SSM](https://github.com/aws/amazon-ssm-document-language-service/) and for [ASL](https://github.com/aws/amazon-states-language-service). 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.

Start by locating the existing SAM and CFN integration in the AWS Toolkit VS Code repository, then compare it with the SSM and ASL language-server wrappers described in the issue. The work is complete when SSM and ASL follow the same YAML extension pattern rather than providing separate built-in wrappers.

Written by the indexing model from the issue text.