feat: add auth.format for formatting credentials

[Phillip9587]

This pull request introduces a new feature to the basic-auth library, adding a method to format credentials into a basic auth header string.

[Phillip9587]

@blakeembrey I rebased and added your suggestions to disallow colons in username and control characters in both username and password.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (08b56ca) to head (af1d90b).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #65   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           21        55   +34     
  Branches         6        20   +14     
=========================================
+ Hits            21        55   +34     

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[Phillip9587]

Maybe we should merge this before #95 so that i can also add a cross-environment encodeBase64() in #95.