Set merge option to true in WebpackAssetsManifest (#800)

When used with bin/shakapacker, merge must be true in all environments
to support users with multiple client configurations.

### Summary

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### Pull Request checklist

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- [ ] Add/update test to cover these changes
- [ ] Update documentation
- [ ] Update CHANGELOG file

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Fixed**
* Manifest merging for multi-client builds now applies across all
environments, preventing duplicated or missing asset entries during
builds.

* **Chores**
* Updated asset manifest behavior to consistently merge manifests,
ensuring uniform asset bundling and predictable build outputs.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Justin Gordon <justin@shakacode.com>

Author: Judahmeek

CHANGELOG.md

@@ -9,6 +9,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Extended manifest merging for multiple client configurations to all environments. [PR #800](https://github.com/shakacode/shakapacker/pull/800) by [Judahmeek](https://github.com/Judahmeek).
+
 Changes since the last non-beta release.
 
 ### Added

package/plugins/webpack.ts

@@ -5,7 +5,7 @@ const { requireOrError } = require("../utils/requireOrError")
 const WebpackAssetsManifest = requireOrError("webpack-assets-manifest")
 const webpack = requireOrError("webpack")
 const config = require("../config") as Config
-const { isProduction, isDevelopment } = require("../env")
+const { isProduction } = require("../env")
 const { moduleExists } = require("../utils/helpers")
 
 const getPlugins = (): unknown[] => {
@@ -17,7 +17,7 @@ const getPlugins = (): unknown[] => {
   const plugins = [
     new webpack.EnvironmentPlugin(process.env),
     new WebpackAssetsManifestConstructor({
-      merge: isDevelopment,
+      merge: true,
       entrypoints: true,
       writeToDisk: true,
       output: config.manifestPath,