Add placeholder component

[mallardduck]

This PR allows for using a placeholder element in a form. The intention of this is to provide a UX similar to what LinkedIn does here:

So with this PR, if this UX were needed you could do:

  Components\Grid::make([
      Components\Toggle::make('is_active')
          ->label('Currently working this role')
          ->default(true)
          ->dependable()
          ->stacked(),
      Components\DatePicker::make('started_at'),
// Our conditional field and equivalent placeholder
      Components\DatePicker::make('departed_at')
          ->when(fn ($record) => $record->is_active !== true),
      Components\Placeholder::make('departed_at', 'Present')
          ->when(fn ($record) => $record->is_active === true), // Note the callback logic here is inverted from above.
  ])->columns(3),

The end results would be:

[danharrin]

Thanks!