Solve shows files empty if they are not in `_files`

[AriPerkkio]

Describe the bug

If _solution doesn't contain every single file from the _files, they are shown as empty when Solve button is clicked. In setup below first.js is shown as empty file when Solve is clicked.

├── _files
│   ├── first.js
│   └── second.js
│
└── _solution
    └── second.js

Steps to reproduce

Locally with TutorialKit repo:

1. Add packages/template/src/content/tutorial/1-basics/1-introduction/2-foo/_solution/src/index.html
2. Open tutorial and press solve
3. All other files are shown as empty

Expected behavior

Solution should inherit files from lesson files (_files).

Platform

• TutorialKit version: 0.0.3