chore: introduces Makefile for consistent command usage

[Justintime50]

Description

Retains current composer commands while providing the more traditional "EasyPost" Makefile targets as well that our other libs provide.

Updated the README with new command instructions.

Testing

• Ran each of these commands to ensure they work as intended

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)