docs(Slider): use composition api

src/slider/README.md

@@ -19,18 +19,19 @@ app.use(Slider);
 ```
 
 ```js
+import { ref } from 'vue';
 import { Toast } from 'vant';
 
 export default {
-  data() {
+  setup() {
+    const value = ref(50);
+    const onChange = (value) => {
+      Toast('Current value: ' + value);
+    };
     return {
-      value: 50,
+      value,
+      onChange,
     };
-  },
-  methods: {
-    onChange(value) {
-      Toast('Current value：' + value);
-    },
   },
 };
 ```
@@ -44,19 +45,20 @@ Add `range` attribute to open dual thumb mode.
 ```
 
 ```js
+import { ref } from 'vue';
 import { Toast } from 'vant';
 
 export default {
-  data() {
+  setup() {
+    // value must be an Array
+    const value = ref([10, 50]);
+    const onChange = (value) => {
+      Toast('Current value: ' + value);
+    };
     return {
-      // value must be an Array
-      value: [10, 50],
+      value,
+      onChange,
     };
-  },
-  methods: {
-    onChange(value) {
-      Toast('current value：' + value);
-    },
   },
 };
 ```
@@ -123,19 +125,21 @@ export default {
 ```
 
 ```js
+import { ref } from 'vue';
 import { Toast } from 'vant';
 
 export default {
-  data() {
+  setup() {
+    const value = ref(50);
+    const value2 = ref([10, 50]);
+    const onChange = (value) => {
+      Toast('Current value: ' + value);
+    };
     return {
-      value: 50,
-      value2: [10, 50],
+      value,
+      value2,
+      onChange,
     };
-  },
-  methods: {
-    onChange(value) {
-      Toast('Current value：' + value);
-    },
   },
 };
 ```