Set up versioning

[dougollerenshaw]

Sets up versioning and mechanisms for auto-incrementing the version.

Addresses #7

Uses bumpversion combined with github actions to automatically update the version when the PR is merged and to add a preview of the expected version number in the PR comment. To use this system:

• For most changes, you don't need to do anything special. The version will automatically bump at the patch level.
• For minor changes, use a commit message starting with feat: or include #minor in the PR title.
• For major changes, include BREAKING CHANGE: in the commit body or #major in the PR title.
• If you merge a PR and realize you wanted a different version bump, you can manually push a commit to main with a message like bump: version 1.2.3 → 1.3.0 to adjust as needed.