Migrate spacing values to particles

[james-nash]

Is your feature request related to a problem? Please describe.
Currently, the SASS code in @buildit/gravity-ui-web defines a number of spacing values that are used throughout the UI (the $grav-sp-*) values.

They should be moved from there into this repo as design tokens. That way, we can begin to export and publish them into other formats to support platforms beyond just the web.

Describe the solution you'd like

• Add new spacing design tokens in this repo
• Update the StyleDictionary config and scripts to export them as suitable SASS vars & TS vars (and anything else we migth support)
• Update the gravity-ui-web repo to:
  • Depend on the new version of @buildit/gravity-particles that contains the spacing values
  • Update the SASS code so that it no longer contains any $grav-sp-* variable declarations and instead imports those vars from @buildit/gravity-particles.