feat: accept function for webpackConfig

Using a function and passing existing config to webpackConfig opens
many possibilities than just using webpackMerge out of the box.

See #7

packages/scripts/@types/webpack-merge.d.ts

@@ -1,6 +1,19 @@
 declare module 'webpack-merge' {
 	import * as webpack from 'webpack';
+	interface customizeArray {
+		(a:any[], b:any[], key:string):any[]
+	}
+	interface customizeObject {
+		(a:Object, b:Object, key:string):Object
+	}
+	interface merge {
+		// Main callable function interface
+		(...configs: webpack.Configuration[]):webpack.Configuration;
+		(configs:webpack.Configuration[]):webpack.Configuration;
+	}
 	function merge(...configs: webpack.Configuration[]):webpack.Configuration;
+	function merge(configs:webpack.Configuration[]):webpack.Configuration;
+	function merge(customizer:{customizeArray:customizeArray, customizeObject:customizeObject}):merge;
 	namespace merge {
 		function smart(...configs: webpack.Configuration[]):webpack.Configuration;
 	}

packages/scripts/src/config/CreateWebpackConfig.ts

@@ -182,7 +182,12 @@ export class CreateWebpackConfig {
 		// Merge options if needed
 		// Loose comparison because it could very well be undefined
 		if (file.webpackConfig != null) {
-			config = webpackMerge(config, file.webpackConfig);
+			// If it is a function
+			if (typeof file.webpackConfig === 'function') {
+				config = file.webpackConfig(config, webpackMerge);
+			} else {
+				config = webpackMerge(config, file.webpackConfig);
+			}
 		}
 
 		return config;

packages/scripts/src/config/project.config.default.ts

@@ -1,4 +1,5 @@
 import webpack from 'webpack';
+import merge from 'webpack-merge';
 
 import { PresetOptions } from '@wpackio/babel-preset-base/lib/preset';
 
@@ -30,7 +31,12 @@ export interface EntryConfig {
 export interface FileConfig {
 	name: string;
 	entry: EntryConfig;
-	webpackConfig?: webpack.Configuration;
+	webpackConfig?:
+		| webpack.Configuration
+		| ((
+				config: webpack.Configuration,
+				api: merge
+		  ) => webpack.Configuration);
 }
 
 export type webpackOptionsOverrideFunction = (