feat(VFileInput): add placeholder and persistent-placeholder props

[sridhar-3009]

Summary

Fixes #19837 — adds placeholder and persistent-placeholder props to v-file-input, similar to v-text-field.

Changes

• VFileInput.tsx: add placeholder: String and persistentPlaceholder: Boolean props; render placeholder text when no file is selected; include persistentPlaceholder in isActive so the label floats
• VFileInput.sass: add .v-file-input__placeholder style
• _variables.scss: add $file-input-placeholder-color SASS variable

Usage

<v-file-input
  label="Upload documents"
  placeholder="Choose one or more files"
  persistent-placeholder
></v-file-input>

Test plan

• Add placeholder to v-file-input — text should appear when no file is chosen
• Verify text disappears after a file is selected
• Verify persistent-placeholder keeps the label floated so placeholder remains visible
• Verify no visual regression when placeholder is omitted

[J-Sek]

• rebase and target dev branch
• ideally we could overlay text input on top, so universal styles for ::placeholder would match