Add Mix task to bump semantic versioning

[Kavignon]

Description

Introduce a Mix task to automate the process of incrementing the semantic version of the template. This task will ensure consistent versioning following the Semantic Versioning (SemVer) standard (MAJOR.MINOR.PATCH), simplifying version management for maintainers.

Tasks

• Create a VERSION file in the root of the repository, initialized with 0.1.0.
• Implement a Mix task (mix version.bump) with the following features:
  • Accept a single argument: patch, minor, or major.
  • Read the current version from the VERSION file.
  • Increment the specified version segment.
  • Write the updated version back to the VERSION file.
• Handle invalid arguments gracefully by printing an error message and exiting with a non-zero status code.
• Add a test module for the Mix task in test/mix/tasks/.
• Update the README with instructions on using the mix version.bump task.
• Update the version field in mix.exs with the new version.