feat(buildEs5): add "prod" as an option for config.buildEs5

In Stencil v2, the default is to not build es5 in production, whereas v1 did. Setting buildEs5 to "prod" will only build es5 in production mode, which is the same as to how v1 worked by default.

src/compiler/config/test/validate-config.spec.ts

@@ -86,13 +86,34 @@ describe('validation', () => {
       expect(config.buildEs5).toBe(true);
     });
 
+    it('set buildEs5 true, dev mode', () => {
+      userConfig.devMode = true;
+      userConfig.buildEs5 = true;
+      const { config } = validateConfig(userConfig);
+      expect(config.buildEs5).toBe(true);
+    });
+
     it('prod mode, set modern and es5', () => {
       userConfig.devMode = false;
       userConfig.buildEs5 = true;
       const { config } = validateConfig(userConfig);
       expect(config.buildEs5).toBe(true);
     });
 
+    it('build es5 when set to "prod" and in prod', () => {
+      userConfig.devMode = false;
+      userConfig.buildEs5 = 'prod';
+      const { config } = validateConfig(userConfig);
+      expect(config.buildEs5).toBe(true);
+    });
+
+    it('do not build es5 when set to "prod" and in dev', () => {
+      userConfig.devMode = true;
+      userConfig.buildEs5 = 'prod';
+      const { config } = validateConfig(userConfig);
+      expect(config.buildEs5).toBe(false);
+    });
+
     it('prod mode default to only modern and not es5', () => {
       userConfig.devMode = false;
       const { config } = validateConfig(userConfig);

src/compiler/config/validate-config.ts

@@ -41,7 +41,7 @@ export const validateConfig = (userConfig?: Config) => {
   config.extras.initializeNextTick = !!config.extras.initializeNextTick;
   config.extras.tagNameTransform = !!config.extras.tagNameTransform;
 
-  config.buildEs5 = !!config.buildEs5;
+  config.buildEs5 = config.buildEs5 === true || (!config.devMode && config.buildEs5 === 'prod');
 
   setBooleanConfig(config, 'minifyCss', null, !config.devMode);
   setBooleanConfig(config, 'minifyJs', null, !config.devMode);

src/declarations/stencil-public-compiler.ts

@@ -115,16 +115,13 @@ export interface StencilConfig {
   rollupConfig?: RollupConfig;
 
   /**
-   * Sets if the ES5 build should be generated or not. It defaults to `false` in dev mode, and `true` in
-   * production mode. Notice that Stencil always generates a modern build too, whereas this setting
-   * will either disable es5 builds entirely with `false`, or always create es5 builds (even in dev mode)
-   * when set to `true`. Basically if the app does not need to run on legacy browsers
-   * (IE11 and Edge 18 and below), it's safe to set `buildEs5` to `false`, which will also speed up
-   * production build times. In addition to not creating es5 builds, apps may also be interested in
-   * disabling any unnecessary runtime when support legacy browsers. See
-   * [https://stenciljs.com/docs/config-extras](/docs/config-extras) for more information.
-   */
-  buildEs5?: boolean;
+   * Sets if the ES5 build should be generated or not. Stencil generates a modern build without ES5,
+   * whereas this setting to `true` will also create es5 builds for both dev and prod modes. Setting
+   * `buildEs5` to `prod` will only build ES5 in prod mode. Basically if the app does not need to run
+   * on legacy browsers (IE11 and Edge 18 and below), it's safe to not build ES5, which will also speed
+   * up build times. Defaults to `false`.
+   */
+  buildEs5?: boolean | 'prod';
 
   /**
    * Sets if the JS browser files are minified or not. Stencil uses `terser` under the hood.