Add bias for planar shadow

[troublemaker52025]

Re: #

Changelog

---

Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

---

Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.

[troublemaker52025]

@cocos-robot run test cases

[github-actions]

Interface Check Report

! WARNING this pull request has changed these public interfaces:

@@ -11935,8 +11935,14 @@
                  */
                 get distance(): number;
                 set distance(val: number);
                 /**
+                 * @en Positional offset values in planar shading calculations.
+                 * @zh 平面阴影计算中的位置偏移值。
+                 */
+                get planeBias(): number;
+                set planeBias(val: number);
+                /**
                  * @en Shadow color.
                  * @zh 阴影颜色。
                  */
                 get shadowColor(): math.Color;
@@ -11975,8 +11981,9 @@
                 protected _instancingMaterial: Material | null;
                 protected _enabled: boolean;
                 protected _type: number;
                 protected _distance: number;
+                protected _planeBias: number;
                 protected _normal: math.Vec3;
                 protected _shadowColor: math.Color;
                 protected _size: math.Vec2;
                 protected _shadowMapDirty: boolean;
@@ -26431,8 +26438,14 @@
          */
         set planeHeight(val: number);
         get planeHeight(): number;
         /**
+         * @en Positional offset values in planar shading calculations.
+         * @zh 平面阴影计算中的位置偏移值。
+         */
+        set planeBias(val: number);
+        get planeBias(): number;
+        /**
          * @en get or set shadow max received
          * @zh 获取或者设置阴影接收的最大光源数量
          */
         set maxReceived(val: number);
@@ -26446,8 +26459,9 @@
         protected _enabled: boolean;
         protected _type: number;
         protected _normal: math.Vec3;
         protected _distance: number;
+        protected _planeBias: number;
         protected _shadowColor: math.Color;
         protected _maxReceived: number;
         protected _size: math.Vec2;
         protected _resource: renderer.scene.Shadows | null;