RichText: support preview content in the editor

[WenhaiLin]

No description provided.

[github-actions]

Code Size Check Report

Wechat (WASM)	Before	After	Diff
2D Empty (legacy pipeline)	1001924 bytes	1001924 bytes	✅ 0 bytes
2D All (legacy pipeline)	2662943 bytes	2663170 bytes	⚠️ +227 bytes
2D All (new pipeline)	2750172 bytes	2750399 bytes	⚠️ +227 bytes
(2D + 3D) All	10003554 bytes	10003781 bytes	⚠️ +227 bytes

Web (WASM + ASMJS)	Before	After	Diff
(2D + 3D) All	16915826 bytes	16916053 bytes	⚠️ +227 bytes

Interface Check Report

! WARNING this pull request has changed these public interfaces:

@@ -2886,8 +2886,17 @@
         get string(): string;
         set string(value: string);
         /**
          * @en
+         * Preview content of RichText in the editor.
+         *
+         * @zh
+         * 富文本在编辑器里的预览文本内容。
+         */
+        get preview(): string;
+        set preview(value: string);
+        /**
+         * @en
          * Horizontal Alignment of each line in RichText.
          *
          * @zh
          * 文本内容的水平对齐方式。
@@ -3006,8 +3015,9 @@
          * @zh 文本垂直对齐类型。
          */
         static VerticalAlign: typeof VerticalTextAlignment;
         protected _lineHeight: number;
+        protected _preview: string;
         protected _string: string;
         protected _horizontalAlign: HorizontalTextAlignment;
         protected _verticalAlign: VerticalTextAlignment;
         protected _fontSize: number;