update(examples): Add validation and zoom pan helper example

examples/Validation/CustomInput.vue

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+<script lang="ts" setup>
+import { Position, Handle, Connection } from '~/index'
+
+const isValidConnection = (connection: Connection) => connection.target === 'B'
+</script>
+<template>
+  <div>Only connectable with B</div>
+  <Handle type="source" :position="Position.Right" :is-valid-connection="isValidConnection" />
+</template>

examples/Validation/CustomNode.vue

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+<script lang="ts" setup>
+import { Position, Handle, Connection, NodeProps } from '~/index'
+
+interface CustomNodeProps extends NodeProps {
+  id: string
+}
+
+const props = defineProps<CustomNodeProps>()
+
+const isValidConnection = (connection: Connection) => connection.target === 'B'
+</script>
+<template>
+  <Handle type="target" :position="Position.Left" :is-valid-connection="isValidConnection" />
+  <div>{{ props.id }}</div>
+  <Handle type="source" :position="Position.Right" :is-valid-connection="isValidConnection" />
+</template>

examples/Validation/ValidationExample.vue

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+<script lang="ts" setup>
+import CustomInput from './CustomInput.vue'
+import CustomNode from './CustomNode.vue'
+import Flow, {
+  addEdge,
+  Handle,
+  Connection,
+  Position,
+  Elements,
+  Edge,
+  OnConnectStartParams,
+  NodeProps,
+  FlowInstance,
+  NodeType,
+} from '~/index'
+
+import './validation.css'
+
+const initialElements: Elements = [
+  { id: '0', type: 'custominput', position: { x: 0, y: 150 } },
+  { id: 'A', type: 'customnode', position: { x: 250, y: 0 } },
+  { id: 'B', type: 'customnode', position: { x: 250, y: 150 } },
+  { id: 'C', type: 'customnode', position: { x: 250, y: 300 } },
+] as Elements
+
+const onLoad = (reactFlowInstance: FlowInstance) => reactFlowInstance.fitView()
+const onConnectStart = ({ nodeId, handleType }: OnConnectStartParams) => console.log('on connect start', { nodeId, handleType })
+const onConnectStop = (event: MouseEvent) => console.log('on connect stop', event)
+const onConnectEnd = (event: MouseEvent) => console.log('on connect end', event)
+
+const elements = ref<Elements>(initialElements)
+const onConnect = (params: Connection | Edge) => {
+  console.log('on connect', params)
+  elements.value = addEdge(params, elements.value)
+}
+const nodeTypes: Record<string, NodeType> = {
+  custominput: CustomInput as NodeType,
+  customnode: CustomNode as NodeType,
+}
+</script>
+<template>
+  <Flow
+    :elements="elements"
+    :select-nodes-on-drag="false"
+    class="validationflow"
+    :node-types="nodeTypes"
+    @connect="onConnect"
+    @oad="onLoad"
+    @connect-start="onConnectStart"
+    @connect-stop="onConnectStop"
+    @connect-end="onConnectEnd"
+  />
+</template>

examples/Validation/validation.css

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+.validationflow .vue-flow__node {
+  width: 150px;
+  border-radius: 5px;
+  padding: 10px;
+  color: #555;
+  border: 1px solid #ddd;
+  text-align: center;
+  font-size: 12px;
+}
+
+.validationflow .vue-flow__node-customnode {
+  background: #e6e6e9;
+  border: 1px solid #ddd;
+}
+
+.vue-flow__node-custominput .vue-flow__handle {
+  background: #e6e6e9;
+}
+
+.validationflow .vue-flow__node-custominput {
+  background: #fff;
+
+}
+
+.validationflow .vue-flow__handle-connecting {
+  background: #ff6060;
+}
+
+.validationflow .vue-flow__handle-valid {
+  background: #55dd99;
+}