fix: rerender components with conditional slots when slotting elements after initialization

[jcfranco]

Related Issue: #3686

Summary

This PR updates components that were already using getSlotted for conditionally rendering their named slots to use the interface and utils from the conditionalSlot.ts util.

Notable changes

1. Refactored the conditional slot helpers to ensure components rerender when elements are appended after initialization
2. Enhanced the slots test helper to help test conditional rendering
3. Refactored calcite-panel's footer slot rendering to render both slots†

† Reasoning for 3:

• Both footer and footer-action slot rendering are identical except for the named slot (see https://github.com/Esri/calcite-components/blob/5b4793b7aae448e785fdaa06ad6824eb10cc6715/src/components/calcite-panel/calcite-panel.tsx#L387-L409)
• The usage of ☝️ (see https://github.com/Esri/calcite-components/blob/5b4793b7aae448e785fdaa06ad6824eb10cc6715/src/components/calcite-panel/calcite-panel.tsx#L459) indicates that it is invalid to slot both footer and footer-action elements, so we can render both and rely on the doc (sidebar: if both slots are identical, can we drop one in favor of the other?)
• It makes it easier to set up and use the test helper, otherwise we need a way to let the test helper know how slots are related to each other (I'd like to avoid this if possible)

[driskull]

Nice!

[driskull]

Looks good!

[jcfranco]

Existing test failure is unrelated:

Summary of all failing tests
FAIL src/components/calcite-dropdown/calcite-dropdown.e2e.ts (69.154 s)
  ● calcite-dropdown › opens the dropdown with click, enter, or space › opens when dropdown-trigger is an action

    expected event "calciteDropdownOpen" to have been called 1 times, but was called 2 times

      828 |       await page.waitForChanges();
      829 |       expect(await dropdownWrapper.isVisible()).toBe(true);
    > 830 |       expect(calciteDropdownOpen).toHaveReceivedEventTimes(1);
          |                                   ^
      831 |     });
      832 |   });
      833 |

      at Object.<anonymous> (src/components/calcite-dropdown/calcite-dropdown.e2e.ts:830:35)

cc @Elijbet

All other checks are ✅. Merging!