Add pubspec mod CLI (pm)

[robbecker-wf]

This pull request introduces significant improvements to project automation, documentation, and dependency management. The main changes include adding CI/CD workflows, automating dependency updates, updating SDK constraints, and expanding documentation for both the replace and new pm command-line tools. Additionally, new test fixtures are added to support dependency constraint editing and recursive operations.

Automation and CI/CD:

• Added a GitHub Actions workflow in .github/workflows/ci.yml to automate testing, formatting, and analysis for every push and pull request on main.
• Introduced .github/dependabot.yml to automate dependency updates for Dart packages and GitHub Actions on a weekly schedule.

Dependency management:

• Updated the Dart SDK constraint in pubspec.yaml to >=3.0.0 <4.0.0, added pubspec_manager as a dependency, and included path and test as dev dependencies. Also, replaced the cider section with the new pubmod executable.

Documentation enhancements:

• Expanded README.md with detailed installation instructions for both replace and the new pm tool, including usage examples, command descriptions, and notes on constraint tightening. [1] [2]
• Improved clarity and formatting throughout README.md, including glob syntax links and dotfile handling. [1] [2] [3]

Test fixtures for dependency editing:

• Added multiple fixture pubspec.yaml files under test/fixtures/pubmod/ to support testing of dependency constraint editing, recursive operations, and error handling for malformed files. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Other improvements:

• Added analysis_options.yaml to exclude test fixtures from static analysis.
• Introduced an auto-documented Makefile for building and installing executables with helpful target descriptions.