Base override styles for checkout next

- Base styles for overriding Shopify's new responsive Checkout
- Some minor changes are still happening to the core styles, so this may be updated in future small releases
- Added a btn and btnSecondary color variables to `timber.scss.liquid`

Hoping this can be a solid reference for any theme designer needing to start overriding the default checkout styles.

[Test it out here](https://checkout.shopify.com/4790585/checkouts/f1634f497dd53e1107a7275cb0b9039c)

cc/ @Shopify/fed @carolineschnapp @fredryk 
`!important` is used because placeholder text colour changes for error states. I was testing this out on a dark theme, so it made the placeholder error text the most visible (and seemingly filled in) text on the page. To keep the error state as is, remove `!important`.

Errors already have their own borders, error message, and arrow notifying them so this should be a fine compromise.