feat(tokens)!: prefer lowercase, hyphenated font names in font stacks

BREAKING CHANGE: updates the font-family stacks to remove the provided
value from `@adobe/spectrum-tokens`, and instead use the "older" naming
syntax that Spectrum CSS had used previously.

Author: pfulton

components/tokens/custom-spectrum/custom-vars.css

@@ -35,16 +35,13 @@ governing permissions and limitations under the License.
   --spectrum-animation-ease-out: cubic-bezier(0, 0, 0.40, 1);
   --spectrum-animation-ease-linear: cubic-bezier(0, 0, 1, 1);
 
-  --spectrum-sans-serif-font: var(--spectrum-sans-serif-font-family);
-  --spectrum-sans-font-family-stack: var(--spectrum-sans-serif-font), adobe-clean, 'Source Sans Pro', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Ubuntu, 'Trebuchet MS', 'Lucida Grande', sans-serif;
+  --spectrum-sans-font-family-stack: adobe-clean, 'Source Sans Pro', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Ubuntu, 'Trebuchet MS', 'Lucida Grande', sans-serif;
 
-  --spectrum-serif-font: var(--spectrum-serif-font-family);
-  --spectrum-serif-font-family-stack: var(--spectrum-serif-font), adobe-clean-serif, 'Source Serif Pro', Georgia, serif;
+  --spectrum-serif-font-family-stack: adobe-clean-serif, 'Source Serif Pro', Georgia, serif;
 
   --spectrum-code-font-family-stack: 'Source Code Pro', Monaco, monospace;
 
-  --spectrum-cjk-font: var(--spectrum-cjk-font-family);
-  --spectrum-cjk-font-family-stack: var(--spectrum-cjk-font), adobe-clean-han-japanese, sans-serif;
+  --spectrum-cjk-font-family-stack: adobe-clean-han-japanese, sans-serif;
 
   /* static white / black background color for docs containers */
   --spectrum-docs-static-white-background-color: rgb(15, 121, 125);