add atContainer rule support

[jogibear9988]

Add support for @container

Description

Add parsing and writing support for at-container

Related Issue

#92

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jogibear9988]

@holblin any change to get this merged?

[holblin]

Thanks a lot for your contribution! Sorry I wasn't able to review and merge earlier. A new version should be available soon on npmjs 👍