build: add webpack-merge ^5 to discussion and problem assets

These webpack configs use const { merge } = require('webpack-merge')
which is webpack-merge v5 API. However webpack-merge was only present
as a transitive dep at v4 (pulled in by karma-webpack ^0.13), causing
TypeError: merge is not a function at build time.

Adding it as a direct devDependency at ^5 causes npm to install v5
under each workspace's node_modules, matching what the configs require.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

package-lock.json

@@ -16,6 +16,7 @@
     "imports-loader": "^0.8",
     "webpack": "^5",
     "webpack-cli": "^5",
+    "webpack-merge": "^5",
     "draggabilly": "^3.0.0",
     "edx-ui-toolkit": "^1",
     "jasmine-core": "2.6.4",

src/xblock_discussion/assets/package.json

@@ -20,6 +20,7 @@
   },
   "devDependencies": {
     "copy-webpack-plugin": "^14.0.0",
+    "webpack-merge": "^5",
     "jasmine-core": "2.6.4",
     "jasmine-jquery": "git+https://git@github.com/velesin/jasmine-jquery.git#ebad463d592d3fea00c69f26ea18a930e09c7b58",
     "karma-coverage": "^2",