chore: import upstream snapshot with attribution

docs/md/how_to/javascript/virtual_server/custom.md

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+# Implementing a custom Virtual Server
+
+You can connect Perspective to any data source by implementing the
+`VirtualServerHandler` interface and passing it to `createMessageHandler()`.
+
+For background on virtual servers, see the
+[Virtual Servers overview](../../../explanation/virtual_servers.md).
+
+## Example
+
+```typescript
+import perspective from "@perspective-dev/client";
+import type {
+    VirtualServerHandler,
+    ColumnType,
+    ViewConfig,
+    ViewWindow,
+    VirtualDataSlice,
+} from "@perspective-dev/client";
+
+const handler = {
+    async getHostedTables(): Promise<string[]> {
+        return ["my_table"];
+    },
+
+    async tableSchema(tableId: string): Promise<Record<string, ColumnType>> {
+        return { name: "string", price: "float", date: "date" };
+    },
+
+    async tableSize(tableId: string): Promise<number> {
+        return 1000;
+    },
+
+    async tableMakeView(
+        tableId: string,
+        viewId: string,
+        config: ViewConfig,
+    ): Promise<void> {
+        // Translate `config` (group_by, sort, filter, etc.) into a query
+        // against your data source. Store the query keyed by `viewId`
+        // for later data retrieval.
+    },
+
+    async viewDelete(viewId: string): Promise<void> {
+        // Clean up resources for this view
+    },
+
+    async viewGetData(
+        viewId: string,
+        config: ViewConfig,
+        schema: Record<string, ColumnType>,
+        viewport: ViewWindow,
+        dataSlice: VirtualDataSlice,
+    ): Promise<void> {
+        // Query your data source using `config` and `viewport` for the
+        // row/column window. Push columnar results via `dataSlice.setCol()`.
+    },
+
+    getFeatures() {
+        return {
+            group_by: true,
+            sort: true,
+            filter_ops: {
+                string: ["==", "!=", "contains", "is null", "is not null"],
+                float: ["==", "!=", ">", "<", ">=", "<="],
+            },
+            aggregates: {
+                float: ["sum", "avg", "count", "min", "max"],
+                string: ["count", "any"],
+            },
+        };
+    },
+} satisfies VirtualServerHandler;
+
+// Create a message handler and use it like a worker
+const messageHandler = perspective.createMessageHandler(handler);
+const client = await perspective.worker(messageHandler);
+const table = await client.open_table("my_table");
+document.getElementById("viewer").load(table);
+```