feat(VField): add placeholder slot support

[wxj006007]

Description

Adds a placeholder slot to VField, VTextField, and VTextarea components, allowing users to customize placeholder content with Vue transitions and other slot features.

Closes #20886

Motivation

Currently, the placeholder prop only accepts a string, making it impossible to use Vue transitions or other dynamic content in the placeholder. The existing label slot already supports this pattern, and this PR extends the same approach to placeholder.

Changes

• VField.tsx: Added placeholder slot type definition and rendering logic
• VField.sass: Added styles for .v-field__placeholder
• VTextField.tsx: Suppress native placeholder attribute when slot is provided
• VTextarea.tsx: Same as above
• Tests: Added unit tests for both VTextField and VTextarea placeholder slot behavior

Usage

<v-textarea v-model="input">
  <template #placeholder>
    <v-fade-transition mode="out-in">
      <div :key="placeholderText">{{ placeholderText }}</div>
    </v-fade-transition>
  </template>
</v-textarea>