feat(selectable-tile): add SelectableTileGroup component

[metonym]

Closes #439

Add SelectableTileGroup component for parity with TileGroup used to manage radio tiles.

Changes

• Added SelectableTileGroup component that wraps multiple SelectableTile components in a fieldset, managing an array of selected values and dispatching select/deselect events
• Modified SelectableTile to detect and integrate with SelectableTileGroup context when present, automatically syncing selection state and using shared name attributes

<script>
  import { SelectableTile, SelectableTileGroup } from "carbon-components-svelte";
  
  let selected = [];
</script>

<SelectableTileGroup bind:selected legend="Select options">
  <SelectableTile value="option1">Option 1</SelectableTile>
  <SelectableTile value="option2">Option 2</SelectableTile>
  <SelectableTile value="option3">Option 3</SelectableTile>
</SelectableTileGroup>

[metonym]

Released in v0.94.0