Feature/add conatiner queries

[jackmcpickle]

Addresses #223 by adding container mixin

Currently some issue with sass-true and container mixin. Maybe a bug?

[jackmcpickle]

@andreimoment Review?

[andreimoment]

@jackmcpickle thank you for submitting these. I noticed a few things:

• I couldn't run the tests - adding @container to the tests results in errors - I see you've commented out that line.
• When the @container tests run OK, I recommend:
  • adding tests using the new container units
  • adding a test for the "named: " parameter
• I did not see any additions pertaining to the "named: ..." parameter.

Note, I am not a maintainer for this - just someone who suggested adding the feature. I apologize for not acting on this sooner - SCSS is not my forte and it will take me a bit of time to understand all of the code and propose an update.

Thank you for acting swiftly and moving this in the right direction.

[jackmcpickle]

@andreimoment all good. I thought I'd just start with the unnamed @container queries for now.