feat: thread enablePrivateMethods through rollup plugin and enable in playground

Add enablePrivateMethods to RollupLwcOptions and forward it to the
compiler's transformSync call. Enable it in the playground rollup
config and add a private method to the counter component for testing.

Made-with: Cursor

Author: a-chabot

packages/@lwc/rollup-plugin/src/index.ts

@@ -74,6 +74,7 @@ export interface RollupLwcOptions {
      * @example '@salesforce/featureFlag/name'
      */
     componentFeatureFlagModulePath?: string;
+    enablePrivateMethods?: boolean;
 }
 
 const PLUGIN_NAME = 'rollup-plugin-lwc-compiler';
@@ -198,6 +199,7 @@ export default function lwc(pluginOptions: RollupLwcOptions = {}): Plugin {
         apiVersion,
         defaultModules = DEFAULT_MODULES,
         componentFeatureFlagModulePath,
+        enablePrivateMethods,
     } = pluginOptions;
 
     return {
@@ -395,6 +397,7 @@ export default function lwc(pluginOptions: RollupLwcOptions = {}): Plugin {
                 targetSSR,
                 ssrMode,
                 componentFeatureFlagModulePath,
+                enablePrivateMethods,
             });
 
             if (warnings) {

playground/rollup.config.js

@@ -21,7 +21,7 @@ export default (args) => {
                 'process.env.NODE_ENV': JSON.stringify(__ENV__),
                 preventAssignment: true,
             }),
-            lwc(),
+            lwc({ enablePrivateMethods: true }),
             args.watch &&
                 serve({
                     open: false,

playground/src/modules/x/counter/counter.js

@@ -7,8 +7,13 @@ export default class extends LightningElement {
 
     increment() {
         this.counter++;
+        this.#privateMethod();
     }
     decrement() {
         this.counter--;
     }
+
+    #privateMethod() {
+        this.counter++;
+    }
 }