feat(VSelectionControl): add description prop below label

[jaskeerat911]

Description

Adds a description prop and #description slot to VSelectionControl,
rendering text below the label with a font-size between label and hint.

Closes #21773

Changes

• Added description prop to makeVSelectionControlProps
• Added v-selection-control__label-wrapper to stack label + description
• Added v-selection-control__description CSS class
• Added SASS variables: $selection-control-description-font-size,
  $selection-control-description-color
• Supports both prop and slot usage

Playground

    <v-checkbox
        label="Accept terms and conditions"
        description="You must read and agree before proceeding"
        hint="View full terms at example.com"
        persistent-hint
    />

    <!-- Slot usage with custom markup -->
    <v-checkbox label="Subscribe to newsletter">
        <template #description>
            Weekly updates only. <a href="#">Unsubscribe anytime</a>
        </template>
    </v-checkbox>

    <!-- Without description — verify existing behaviour unchanged -->
    <v-checkbox label="No description here" />