fix(component-library): treat slot content that renders only comments as empty

[Arnold Stoba (arnoldstoba)]

What?

hasSlotContent treats slot content that renders only comments as empty, also inside v-for fragments. It also accepts slots typed with defineSlots().

Why?

Filtering a list with <template v-for> and v-if, as the Vue docs recommend, renders only comments when every item is hidden. hasSlotContent counted that as content, so components that render an optional slot's wrapper only when it has content showed an empty wrapper. For example, with the upcoming mt-message, a user message with only an image attachment rendered an empty text bubble:

<mt-message from="user">
  <template #attachments>…</template>
  <template v-for="(text, index) in texts" :key="index">
    <mt-text v-if="text.text" size="xs">{{ text.text }}</mt-text>
  </template>
</mt-message>

Vue applies the same rule when it decides whether <slot> shows its fallback content.

How?

One recursive check: comments, empty text and fragments whose children are all empty count as empty.

Testing?

slot.spec.ts renders real templates: a hidden v-if, a v-for with every item hidden and one with an item shown.

Anything Else?

No visible change for existing components: mt-base-field is the only caller today, and every field fills its error slot with a single v-if, so there is no changeset. #1389 and #1399 carry this commit and need a rebase after this is merged.

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