Allow the user to add files

[AriPerkkio]

Discussed in #204

^{Originally posted by GaetanRdn August 1, 2024}

Like in Stackblitz, the possibility to add file from the file tree could be a great feature.

We could create bigger exercices, more difficult.

In my case, I am preparing a tutorial for Angular. And with it, we quickly need to create new files.

This feature request is about adding new files using the file tree. This is something we can implement right away.

In #208 there is a feature request for adding files via terminal.