Add manual version bump, release, and PyPI publish workflow

[jaseemjaskp]

Summary

• Replace the release-triggered publish workflow with a workflow_dispatch trigger
• Add a dropdown input to select version bump type (major/minor/patch)
• Automatically bump version in src/unstract/llmwhisperer/__init__.py, commit, tag, and push
• Create a GitHub release with auto-generated notes using gh CLI
• Build and publish to PyPI after release creation

Test plan

• Trigger the workflow manually from the Actions tab with each bump type (patch, minor, major)
• Verify the version in __init__.py is correctly bumped
• Verify a GitHub release is created with the correct tag
• Verify the package is published to PyPI