[Layout] - Implemented VStack and HStack (Resolves #22)

[matthew-carroll]

[Layout] - Implemented VStack and HStack (Resolves #22)

I added an export for Material Colors and Color so that we can access them from swift_ui without importing material in client code.

I added golden_toolkit for golden tests. The pixel flakiness will be a problem, but goldens seem like the right tool for the job for testing layout behaviors.

[matthew-carroll]

FYI @angelosilvestre @suragch - I'm gonna come back to this project now and then when I can. The UX Lead effort didn't work out because there simply wasn't enough follow through across the board. Therefore, I'm gonna try to contribute obvious widgets where I can. I welcome either of you to do the same, if you'd like. Perhaps once a critical foundation is built it will attract more capable and committed contributors.

If either of you do choose to contribute widgets, please be sure to file a ticket for that widget and assign it to yourself so that I don't accidentally work on the same thing.

[suragch]

@matthew-carroll
Sounds good. I see you're assigned to Text (#2). I'll look for something easy to get my feet wet and then move on to others. Maybe the Rectangle you mentioned in this PR.