feat: add `template.visibleFiles` option

[AriPerkkio]

• Adds template.visibleFiles option that can be used to toggle files from template visible in lessons
• Example usage: https://github.com/AriPerkkio/tutorial-vite-plugin/pull/15
• Closes https://github.com/stackblitz/tutorialkit/issues/393

[bolt-new-by-stackblitz[bot]]

Run & review this pull request in StackBlitz Codeflow.

[AriPerkkio]

Running into https://github.com/stackblitz/tutorialkit/issues/167 here too. When "Solve" is clicked, files from template are shown as empty. Only files in _solution are shown correctly.

~~Need to fix https://github.com/stackblitz/tutorialkit/issues/167 first.~~ ✅

[AriPerkkio]

Tested manually with https://github.com/AriPerkkio/tutorial-vite-plugin/pull/15 - the amount of removed duplicate files is a lot!

As this changes quite a lot how editor's files are resolved, I think we should wait for @Nemikolh or @SamVerschueren to review this next week.