Create css var lib

[chrisvdm]

Create css var lib to store constants
example:

{
  --padding: var(--size-3);
}

Example of constants: padding, typography, colour-palette, site-content-width

[jtoar]

@chrisvdm Even though it's small, should we close this one now that we've got some code in https://github.com/redwoodjs/example-store-stripe/blob/main/web/src/index.css?

example-store-stripe/web/src/index.css

Lines 3 to 6 in f04d178

	:root {
	--link: var(--pink-4);
	--padding: var(--size-3);
	}

#119 also adds a few. We can keep updating our CSS changes in #52.