feat: add CLI

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Add an npx-style cli #2
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Adds a basic bin/index.js that supports one username.

💖

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.34%. Comparing base (cba531f) to head (6f6aad5).
Report is 9 commits behind head on main.

Files	Patch %	Lines
src/cli/githubUsernameToEmailsCli.ts	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
+ Coverage   96.13%   96.34%   +0.21%     
==========================================
  Files           7        8       +1     
  Lines         181      219      +38     
  Branches       13       19       +6     
==========================================
+ Hits          174      211      +37     
- Misses          7        8       +1     

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