refactor StyleOutlet component

[yoli799480165]

Breaking Changes

• StyleOutlet was supported only in .NET 8.0 and above; after refactoring it supports all frameworks.
• UseStyleRegister method in StyleHelper no longer needs to return a RenderFragment.

Originally, styles were injected within the component:

<div></div>
@_styleContent

@code{
  // A RenderFragment was required to trigger updates.
  private RenderFragment _styleContent;
  protected override void OnInitialized()
  {
    _styleContent = StyleHelper.UseStyleRegister();
  }
}

After refactoring:

<div></div>

@code{
  protected override void OnInitialized()
  {
    // styles are now injected directly, RenderFragment is no longer required.
    StyleHelper.Register();
  }
}