chore: use env variable to choose the webpack version to test

jest.config.js

@@ -1,6 +1,24 @@
-module.exports = {
+const config = {
   preset: 'ts-jest',
   testTimeout: 10000,
   testEnvironment: 'node',
   testPathIgnorePatterns: ['<rootDir>/dist/', '<rootDir>/node_modules/'],
 }
+
+if (process.env.WEBPACK5) {
+  module.exports = {
+    ...config,
+
+    globals: {
+      'ts-jest': {
+        diagnostics: false,
+      },
+    },
+    moduleNameMapper: {
+      '^webpack$': 'webpack5',
+      '^webpack/(.*)': 'webpack5/$1',
+    },
+  }
+} else {
+  module.exports = config
+}

package.json

@@ -13,7 +13,7 @@
     "build": "tsc",
     "pretest": "tsc",
     "test": "jest --coverage",
-    "test:webpack5": "jest -c jest-webpack5.config.js --coverage",
+    "test:webpack5": "WEBPACK5=true jest -c --coverage",
     "dev-example": "webpack-dev-server --config example/webpack.config.js --inline --hot",
     "build-example": "rm -rf example/dist && webpack --config example/webpack.config.js --env.prod",
     "lint": "prettier --write --parser typescript \"{src,test}/**/*.{j,t}s\"",