feat(io_file): Add POSIX implementation for createTemporaryDirectory

[brianquinlan]

Adds the createTemporaryDirectory(String template) method to the FileSystem interface.

Implements this method for the PosixFileSystem using the POSIX mkdtemp function via dart:ffi and package:stdlibc. This allows creating unique temporary directories based on a provided template string ending in "XXXXXX".

Includes unit tests for the POSIX implementation, verifying:

• Successful directory creation and path generation.

• Error handling for invalid templates (non-existent parent path, missing "XXXXXX" suffix).

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