feat(appcomposer): add CodeLens link to Application Composer

[JLargent42]

Problem

Users have reported difficulty finding Application Composer in the Toolkit.

Solution:

This adds an "Open with Application Composer" link using CodeLens to the start of detected CloudFormation / SAM templates.

This PR also removes the requirement that CFN templates are YAML, as they can also be JSON. js-yaml correctly parses JSON, so the code changes are minor.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.