refactor: don't generate duplicate virtual code for defineSlots

[johnsoncodehk]

For defineSlots, we can reuse its return type instead of generating duplicate virtual code for template.

For the following vue code.

<script setup lang="ts">
defineSlots<{
  activator?(props: { isActive: boolean }): void
}>();
</script>

<template>
	<!-- ... -->
</template>

Old virtual code

/* script setup part */
// ...
defineSlots<{
  activator?(props: { isActive: boolean }): void
}>();
// ...

/* template part */
var __VLS_slots!: {
  activator?(props: { isActive: boolean }): void
};
// some template virtual codes reference __VLS_slots

New virtual code

/* script setup part */
// ...
const __VLS_slots = defineSlots<{
  activator?(props: { isActive: boolean }): void
}>();
// ...

/* template part */
// some template virtual codes reference __VLS_slots

[johnsoncodehk]

Also looking to do that for defineEmit.