feat(runtime): option for setting terminal open by default

[AriPerkkio]

• Closes Add terminalOpen to frontmatter metadata #132
• Example usage feat: set terminal open in env var lessons AriPerkkio/tutorial-vite-plugin#19
• Adds also test cases that won't be run before test: add tests for ui #241 is merged

Adds new terminal.open option that can be used to set terminal open by default for specific lesson.

---
type: lesson
title: Default
terminal:
  open: true       # <-- Terminal will be open when page is navigated to
  panels: terminal
---

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[AriPerkkio]

#245

[Nemikolh]

Good work on this!! 🎉