Path.withSuffix

Return a new Path with the last suffix (extension) replaced.

Modeled after pathlib's with_suffix:

• newSuffix must be empty (to remove the last suffix) or start with a dot (e.g., ".md").
• If there is no suffix and newSuffix is non-empty, it is appended to the name.
• Only the last suffix is affected; earlier suffixes (e.g., ".tar" in "archive.tar.gz")

remain part of the stem.

• Dotfiles without additional dots (e.g., ".bashrc") are treated as having no suffix;

in that case withSuffix(".txt") yields ".bashrc.txt".