fix: correct slider track margin to react to density level (#3607)

Co-authored-by: Dexter Lesaca <dexterlesaca@JAYACALI-CON.redmond.corp.microsoft.com>
Co-authored-by: Jane Chu <7559015+janechu@users.noreply.github.com>
Co-authored-by: Chris Holt <chhol@microsoft.com>

packages/web-components/fast-components/src/slider/slider.styles.ts

@@ -88,13 +88,13 @@ export const SliderStyles = css`
         right: calc(var(--track-overhang) * 1px);
         left: calc(var(--track-overhang) * 1px);
         align-self: start;
-        margin-top: calc((var(--design-unit) + 2) * 1px);
+        margin-top: calc((var(--design-unit) + calc(var(--density) + 2)) * 1px);
         height: calc(var(--track-width) * 1px);
     }
     :host(.vertical) .track {
         top: calc(var(--track-overhang) * 1px);
         bottom: calc(var(--track-overhang) * 1px);
-        margin-left: calc((var(--design-unit) + 2) * 1px);
+        margin-left: calc((var(--design-unit) + calc(var(--density) + 2)) * 1px);
         width: calc(var(--track-width) * 1px);
         height: 100%;
     }