Add design token functions

[tihuan]

Design token functions are much easier to use!

Example:

1. The theme.ts file change
2. Subsequent whole app refactor https://github.com/chanzuckerberg/single-cell-data-portal/pull/5329

Requirements:

1. Create token functions for all theme tokens
2. Create documentation around usage and examples. NOTE: there are commonly 3 ways to use the token functions:
   1. The most common one. Just use it as is E.g., color: ${textPrimary};
   2. Using it inside a function
   3. Using it to export a css function

[tihuan]

@cathystoli this is the token function ticket I was talking about the other day 😄 Thank you!!