fix(devcontainer): make it possible to run mocha tests in VSCode using different extension

Indeed https://github.com/maty21/mocha-sidebar/issues/181#issuecomment-1016768595 seems to work and pointed me to a better extension.
This works better than maty.vscode-mocha-sidebar which could not find nyc.

.vscode/settings.json

@@ -8,5 +8,7 @@
         "${workspaceFolder}/tst/phpunit.xml",
         "--bootstrap",
         "${workspaceFolder}/tst/Bootstrap.php"
-    ]
+    ],
+    "mochaExplorer.pkgFile": "js/package.json",
+    "mochaExplorer.files": "js/test/*.js",
 }