Update Characteristics spec template

[pseys]

There's an opportunity to streamline the way we document our characteristics within Salt component and pattern specs. It may also be a useful resource for designers outside the core team.

• Add all characteristic tokens as a component in the Salt Spec Utility file -- Designers can then drop in a component and set the props depending on the tokens they need -- Providing tokens through the Spec Utilities will ensure design specs are always up-to-date
• Create a theme switcher component that can be dropped into a page to control theme variables when in prototype mode in Figma -- This removes the need for displaying dark mode examples within the characteristic spec.