Add updateassets command with webapp code detection

- Add updateassets command to update plugin files from embedded assets
- Only include webapp assets if plugin manifest indicates webapp code presence
- Compare file contents before updating to avoid unnecessary writes
- Display count of updated files in completion message

🤖 Generated with [Claude Code](https://claude.ai/code)

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

assets/webapp/babel.config.js

@@ -0,0 +1,46 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+const config = {
+    presets: [
+        ['@babel/preset-env', {
+            targets: {
+                chrome: 66,
+                firefox: 60,
+                edge: 42,
+                safari: 12,
+            },
+            modules: false,
+            corejs: 3,
+            debug: false,
+            useBuiltIns: 'usage',
+            shippedProposals: true,
+        }],
+        ['@babel/preset-react', {
+            useBuiltIns: true,
+        }],
+        ['@babel/typescript', {
+            allExtensions: true,
+            isTSX: true,
+        }],
+        ['@emotion/babel-preset-css-prop'],
+    ],
+    plugins: [
+        '@babel/plugin-proposal-class-properties',
+        '@babel/plugin-syntax-dynamic-import',
+        '@babel/proposal-object-rest-spread',
+        '@babel/plugin-proposal-optional-chaining',
+        'babel-plugin-typescript-to-proptypes',
+    ],
+};
+
+// Jest needs module transformation
+config.env = {
+    test: {
+        presets: config.presets,
+        plugins: config.plugins,
+    },
+};
+config.env.test.presets[0][1].modules = 'auto';
+
+module.exports = config;