Update embedfs implementation

[this-kirke]

Fixes to embedfs implementation:

• Fix path normalization to support billy's absolute path convention ("/")
• Implement missing Lstat method (was returning ErrNotSupported)
• Add proper embed.FS to billy path conversion throughout all methods

Additional test coverage:

• Add dedicated test files for different functionality areas
• Test File interface methods including ReadAt, Close, Lock/Unlock
• Add path normalization and edge case testing
• Test empty file handling and boundary conditions
• Verify proper error handling for read-only operations

Test infrastructure:

• Create embedfs_testdata package for reusable test data
• Resolve import cycles with clean provider pattern
• Add test data including nested directories and various file types
• Organize tests by functionality (fs, dir, file operations)

The embedfs implementation now provides billy.Filesystem compliance for read-only embedded filesystems.

[pjbgf]

@this-kirke thanks for proposing these changes. Please take a look at the feedback below.

[this-kirke]

Hi Paulo! Thank you for the review. I've updated with requested changes; all tests pass.

[pjbgf]

@this-kirke Thanks for the follow up. I just did a full review now, there are a few tests that are redundant which we can remove. But otherwise it LGTM.

Please squash your commits once you are done with the changes below.

[this-kirke]

Thank you for the detailed review! I agree on all points. I think everything should be resolved now; please let me know if there's anything else you notice. We can squash on complete.