feat(astro): support lessons without parts or chapters

[AriPerkkio]

• Closes https://github.com/stackblitz/tutorialkit/issues/304

🚧 WIP

Adds support for defining lessons without chapters or parts.

src/content/tutorial
│
├── meta.md # Tutorial metadata
│
└── lesson-1 # Lesson in root! No part or chapter.
   ├── content.md
   ├── _files
   └── _solution

src/content/tutorial
│
├── meta.md # Tutorial metadata
│
└── part-1
    └── lesson-1 # Lesson in part! No chapter.
        ├── content.md
        ├── _files
        └── _solution

Custom order can be defined using lessons in metadata:

---
type: 'tutorial'
lessons: ['lesson-one', 'lesson-two']
---

---
type: 'part'
lessons: ['lesson-one', 'lesson-two']
---

Mixing structures is not supported and an error is thrown when these are not followed:

• A tutorial can have one-or-many part's or one-or-many lesson's
• A part can have one-or-many chapter's or one-or-many lesson's

However you can have multiple part's, where some contain just chapter's and some contain just lesson's.

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

Run & review this pull request in StackBlitz Codeflow.

[AriPerkkio]

/pkg-pr-new

[AriPerkkio]

/pkg-pr-new

[AriPerkkio]

/pkg-pr-new

⚡️ Your npm packages are published. @tutorialkit/astro: npm i https://pkg.pr.new/@tutorialkit/astro@b57a764 @tutorialkit/runtime: npm i https://pkg.pr.new/@tutorialkit/runtime@b57a764 @tutorialkit/react: npm i https://pkg.pr.new/@tutorialkit/react@b57a764 @tutorialkit/theme: npm i https://pkg.pr.new/@tutorialkit/theme@b57a764 @tutorialkit/types: npm i https://pkg.pr.new/@tutorialkit/types@b57a764

[AriPerkkio]

/pkg-pr-new

⚡️ Your npm packages are published. @tutorialkit/astro: npm i https://pkg.pr.new/@tutorialkit/astro@431145a @tutorialkit/react: npm i https://pkg.pr.new/@tutorialkit/react@431145a @tutorialkit/runtime: npm i https://pkg.pr.new/@tutorialkit/runtime@431145a @tutorialkit/theme: npm i https://pkg.pr.new/@tutorialkit/theme@431145a @tutorialkit/types: npm i https://pkg.pr.new/@tutorialkit/types@431145a

[AriPerkkio]

Thanks for review @Nemikolh, let's get this merged! 💯