feat: Add story tree UI (fix #16)

[hugoattal]

fix: #16

Description

This generate a file tree for stories:

About the configuration:

tree.file

• title (default) use slashes of the story title to create the path
• path use the relative directory path of the story
• function a function that takes an object {title, path} and return an array of string, the last one being the file name

tree.order

• asc (default) alphabetical order in each directory
• function a function that takes two string in argument and return a number to sort files

Additional context

The creation of the tree require two steps:

• On step to create an object with folder being keys, so that it's fast to generate (O(n))
• On step to generate the array tree to keep files in order

If I used directly an array without the object step, it could have been quite slow to check if subfolder already exists (O(n²)).
Though, it might be overengineered, you tell me.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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