[#106] Backend-agnostic `listDirectoryContents`

[DK318]

Description

Problem

At this moment the listDirectoryContents function returns [Text] and in getEntryOrDir we rely on the fact that vault adds a '/' to the directory. But other backends might not behave like this.

Solution

Created new data DirectoryContents which stores 2 lists:

1. list with directories
2. list with entries and changed return type from Maybe [Text] to Maybe DirectoryContents in listDirectoryContents function.

Related issue(s)

• Fixes #106

Fixed #

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