Replace very old built-ins by builtins extensions pack

[marcdumais-work]

I noticed the vscode built-in extensions for the test app are really old. They may carry security issues and can be replaced by a single entry (except clangd, thats not an official "built-in").

It's possible to replace this long list of built-in extensions by a single extension pack [1]. The pack will remain more up--to-date too, since extensions from it are installed at the latest supported version.

Note: a little bug occurred with the last built-ins release, that excluded js-debug and its companion extension, from being part of the pack. It will be fixed next release, but in the meantime, if desired, these extensions will need to be listed like so

[1]: https://open-vsx.org/extension/eclipse-theia/builtin-extension-pack