feat(tray): use token for top and bottom edge to content area

- Use new token tray-top-to-content-area that applies to both the top
  and the bottom; this adds padding-block to the tray. Tray is changed
  to box-sizing border-box so max-block-size and other size properties
  are not changed by the added padding.

components/tray/index.css

@@ -51,6 +51,9 @@ governing permissions and limitations under the License.
   inline-size: 100%;
   max-block-size: calc(100vh - var(--mod-tray-spacing-edge-to-tray-safe-zone, var(--spectrum-tray-spacing-edge-to-tray-safe-zone)));
   margin-block-start: var(--mod-tray-spacing-edge-to-tray-safe-zone, var(--spectrum-tray-spacing-edge-to-tray-safe-zone));
+  padding-block-start: var(--mod-tray-top-to-content-area, var(--spectrum-tray-top-to-content-area));
+  padding-block-end: var(--mod-tray-bottom-to-content-area, var(--spectrum-tray-top-to-content-area));
+  box-sizing: border-box;
   overflow: auto;
   outline: none;
 

components/tray/metadata/mods.md

@@ -1,10 +1,12 @@
 | Modifiable Custom Properties                |
 | ------------------------------------------- |
 | `--mod-tray-background-color`               |
+| `--mod-tray-bottom-to-content-area`         |
 | `--mod-tray-corner-radius`                  |
 | `--mod-tray-entry-animation-delay`          |
 | `--mod-tray-entry-animation-duration`       |
 | `--mod-tray-exit-animation-delay`           |
 | `--mod-tray-exit-animation-duration`        |
 | `--mod-tray-max-inline-size`                |
 | `--mod-tray-spacing-edge-to-tray-safe-zone` |
+| `--mod-tray-top-to-content-area`            |