fix: mutate.Extract path manipulation on Windows

[helsaawy]

mutate.Extract replaces / with \ on Windows (even for non-Windows images) due to the use of filepath.Clean(), and other functions (e.g., filepath.Dir) that ultimately call filepath.Clean on the result.
This is typically not desired, since:

• the flattened image would not be valid on Linux
• Windows can handle / as a path separator, if the result is meant to be extracted on Windows
• the conversion may be non-reversible in the edge case where the image contains a file name with \

Add an imageFS interface to abstract away path manipulation operations (e.g., Dir, Base, Clean, Join), with the default case being filepath.*, but allows using path.* if extracting a non-Windows image on Windows.

Note: extracting a Windows image on Linux is left as is, since neither path nor filepath will be able to appropriately handle the paths if the separator is \.

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