Fix support for long non-residential symlinks

[twiggler]

The targets of non-residential symlinks were not completely read. This is because the size of the symlink header was not taken into account on XFS version 5.

Added a test with a very long symlink and also check number of blocks.

Bonus:

• Modernize typings

Closes #34

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 65.47%. Comparing base (02c5531) to head (c9ca5bf).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dissect/xfs/xfs.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   64.50%   65.47%   +0.96%     
==========================================
  Files           4        4              
  Lines         417      420       +3     
==========================================
+ Hits          269      275       +6     
+ Misses        148      145       -3     

Flag	Coverage Δ
unittests	65.47% <88.88%> (+0.96%)	⬆️

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[Schamper]

While it's maybe a bit nicer to have this file in tests/data, it's more consistent with other dissect projects to have it in the project root instead.

[twiggler]

Apparently there already was a .gitattributes in the project root, so it is also more consistent with this project :)
Fixed.