fix: Make types of File and Folder more explicit

[susnux]

Both inherit from Node which is generic, thus e.g. the mime field is of type string. But for folders the mime can never be something different to 'httpd/unix-directory'.
So the correct type would be the string literal 'httpd/unix-directory'.

The same for the type attribute for File and Folder.

---

⬆️ While this could be seen as an improvement it actually fixes a Typescript issue:

const IFolder = new Folder({ ... })

This I expect to work, but it fails with:

Type 'Folder' is not assignable to type 'IFolder'.
Types of property 'type' are incompatible.
Type 'FileType' is not assignable to type 'FileType.Folder'.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.77%. Comparing base (ca0ed89) to head (19395e5).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1118   +/-   ##
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  Coverage   91.77%   91.77%           
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  Files          23       23           
  Lines         632      632           
  Branches      166      166           
=======================================
  Hits          580      580           
  Misses         45       45           
  Partials        7        7           

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[codecov]

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