[webpack-dev-server] Add httpProxyMiddleware.Filter support to context property of ProxyConfigArrayItem

webpack-dev-server [proxy documentation](https://webpack.js.org/configuration/dev-server/#devserverproxy) gives an example of using Filter function for context property and in reality dev-server supports it. However, it was missing on `@types/webpack-dev-server`.

types/webpack-dev-server/index.d.ts

@@ -28,7 +28,7 @@ declare namespace WebpackDevServer {
 
     type ProxyConfigArrayItem = {
         path?: string | string[];
-        context?: string | string[]
+        context?: string | string[] | httpProxyMiddleware.Filter
     } & httpProxyMiddleware.Config;
 
     type ProxyConfigArray = ProxyConfigArrayItem[];

types/webpack-dev-server/webpack-dev-server-tests.ts

@@ -87,6 +87,9 @@ const c3: WebpackDevServer.Configuration = {
 const c4: WebpackDevServer.Configuration = {
     writeToDisk: (filePath: string) => true,
 };
+const c5: WebpackDevServer.Configuration = {
+    proxy: [{context: (pathname: string) => true}]
+};
 
 // API example
 server = new WebpackDevServer(compiler, config);