[#769] Refactor getters for design tokens

[planktonic]

Fixes #769

Changes

• Adds design-token.get() sass function
• Replaces size.get(), palette.get(), shadow.get(), line-height.get(), custom-property.get(), and font-size.get() uses with the unified design-token.get
• Uses var() at the point of use instead of as part of the outp[ut (as was specified in the issue), because it seems more transparent and expected
• Animation $transition variables are now a map just like the other design tokens, and all uses of the transition variables now use the CSS custom properties for those, matching the other design tokens
• Divides the design tokens into primary & secondary to avoid issues with Sass modules (@using the design tokens in tools/design-token in order to allow validation of a design token’s existence was causing import loops when design-tokens.get() was used in the design-tokens files themselves. Now primary design tokens can’t use the design-token.get() helper)

How to QA

👀 Visual changes

No visual changes

👾 Code changes

Check:

• [ ] The documentation is correct, understandable, and up-to-date.

Preflight checks

• [x] Storybook documentation has been updated
• [x] Fixtures in test/scss/ have been updated
• [x] Your changes have been added to the unreleased section of CHANGELOG.md

[planktonic]

Still TODO:

• [x] Update docs for all each of the design tokens (some done, but check they reflect the current status
• [x] Update the design token overview to reflect the requirements to divide into primary/secondary design tokens