feat(token): expose source of token

[jaredallard]

Adds a new Source field to the token.Token struct, which displays
the source of the token. This is inteded to be used by consumers of this
package for debugging purposes.

As part of this, I also added more tests to help prevent breakages.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.72%. Comparing base (5c49ebc) to head (92f852b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   55.81%   59.72%   +3.90%     
==========================================
  Files          18       18              
  Lines         722      730       +8     
==========================================
+ Hits          403      436      +33     
+ Misses        262      234      -28     
- Partials       57       60       +3     

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