Add warning for big-endian squashfs filesystems

[JSCU-CNI]

This PR adds the reverse endianness of the SquashFS magic header to the squashfs_def definition.

This PR adds a NotImplementedError exception for big-endian 3.x or older SquashFS filesystems.

[Schamper]

Can you also add a unit test ;)

[JSCU-CNI]

Could you enable LFS @Schamper? Currently unable to push.

[Schamper]

Done.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.10%. Comparing base (0887fd2) to head (eec836f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   86.02%   86.10%   +0.07%     
==========================================
  Files           5        5              
  Lines         365      367       +2     
==========================================
+ Hits          314      316       +2     
  Misses         51       51              

Flag	Coverage Δ
unittests	86.10% <100.00%> (+0.07%)	⬆️

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