feat(v2): env variable TERSER_PARALLEL to customize TerserPlugin.parallel (#3497)

* fix(v2): disable terser parallel on CIs

Do not run Terser in parallel when using CircleCI or similar CI environments
to avoid "Error: Call retries were exceeded" errors. For more information
see https://github.com/webpack-contrib/terser-webpack-plugin#parallel

`CI=true` is true for:
- https://docs.gitlab.com/ee/ci/variables/#debug-logging
- https://docs.github.com/en/free-pro-team@latest/actions/reference/environment-variables#default-environment-variables
- https://circleci.com/docs/2.0/env-vars/#built-in-environment-variables
- https://docs.travis-ci.com/user/environment-variables/

* fix(v2): check for possibly undefined env variable

* fix(v2): configurable terser parallelization

* fix(v2): correct terser codedoc

* fix(v2): explicitly test for undefined

* fix(v2): add radix to parseInt

* fix(v2): add missing semicolon

* fix(v2): resolve prettier issues

* fix(v2): resolve remaining prettier issues

packages/docusaurus/src/webpack/base.ts

@@ -38,6 +38,17 @@ export function excludeJS(modulePath: string): boolean {
   );
 }
 
+// See https://github.com/webpack-contrib/terser-webpack-plugin#parallel
+let terserParallel: boolean | number = true;
+if (process.env.TERSER_PARALLEL === 'false') {
+  terserParallel = false;
+} else if (
+  process.env.TERSER_PARALLEL &&
+  parseInt(process.env.TERSER_PARALLEL, 10) > 0
+) {
+  terserParallel = parseInt(process.env.TERSER_PARALLEL, 10);
+}
+
 export function createBaseConfig(
   props: Props,
   isServer: boolean,
@@ -103,7 +114,7 @@ export function createBaseConfig(
           ? [
               new TerserPlugin({
                 cache: true,
-                parallel: true,
+                parallel: terserParallel,
                 sourceMap: false,
                 terserOptions: {
                   parse: {