Upgrade to Textual 5, add release workflows

[edward-jazzhands]

Usage / API changes

• Upgraded to Textual 5.0.0.

Code and project changes

• Renamed Changelog.md to CHANGELOG.md
• Added 2 workflow to .github/workflows:
  • ci-checks.yml - runs Ruff, MyPy, BasedPyright (will add Pytest later)
  • release.yml - Workflow to publish to PyPI and github releases
• Added 2 scripts to .github/scripts:
  • adds .github/scripts/validate_main.sh
  • adds .github/scripts/tag_release.py
• Added 1 new file to root: ci-requirements.txt - this is used by the ci-checks.yml workflow to install the dev dependencies.
• Added basedpyright as a dev dependency to help with type checking. Made the just typecheck command run it after MyPy and set it to 'strict' mode in the config (added [tool.basedpyright] section to pyproject.toml).
• Replaced build and publish commands in the justfile with a single release command that runs the two above scripts and then pushes the new tag to Github
• Workflow update-docs.yml now runs only if the release.yml workflow is successful, so it will only update the docs if a new release is made (Still possible to manually run it if needed, should add a 'docs' tag in the future for this purpose).
• Changed the .python-version file to use 3.9 instead of 3.12.