Fix next-release folder recreation after release

[raulgomis]

Description

Reusing the same core instance programmatically across multiple releases raised a FileNotFoundError when adding subsequent changesets.

Steps to reproduce

    releaser = Semversioner(path=directory_name)
    releaser.add_change("minor", "First")
    releaser.release()
    releaser.add_change("patch", "Second")

Fix

To resolve this, we updated the create_changeset method in storage.py to check for the directory's existence and recreate it if necessary just-in-time.