refactor: move horizontal-layout core CSS to styles folder (#9448)

Author: vursen

packages/horizontal-layout/src/vaadin-horizontal-layout-styles.d.ts renamed to packages/horizontal-layout/src/styles/vaadin-horizontal-layout-styles.d.ts

@@ -3,7 +3,7 @@
  * Copyright (c) 2017 - 2025 Vaadin Ltd.
  * This program is available under Apache License Version 2.0, available at https://vaadin.com/license/
  */
-import { css } from '@vaadin/vaadin-themable-mixin/vaadin-themable-mixin.js';
+import { css } from 'lit';
 
 export const baseStyles = css`
   :host {

packages/horizontal-layout/src/vaadin-horizontal-layout-styles.js renamed to packages/horizontal-layout/src/styles/vaadin-horizontal-layout-styles.js

@@ -9,8 +9,8 @@ import { ElementMixin } from '@vaadin/component-base/src/element-mixin.js';
 import { PolylitMixin } from '@vaadin/component-base/src/polylit-mixin.js';
 import { CSSInjectionMixin } from '@vaadin/vaadin-themable-mixin/css-injection-mixin.js';
 import { ThemableMixin } from '@vaadin/vaadin-themable-mixin/vaadin-themable-mixin.js';
+import { horizontalLayoutStyles } from './styles/vaadin-horizontal-layout-styles.js';
 import { HorizontalLayoutMixin } from './vaadin-horizontal-layout-mixin.js';
-import { horizontalLayoutStyles } from './vaadin-horizontal-layout-styles.js';
 
 /**
  * `<vaadin-horizontal-layout>` provides a simple way to horizontally align your HTML elements.