Add Codecov configuration

[twyatt]

Fixes Codecov coverage reporting by configuring path fixes (which are needed because Codecov doesn't natively understand Kotlin source organization).

The path fix re-writes the path at which Codecov will look for source files, whereas when looking for class com.juul.kronicle.Example Codecov would traditionally look for that class file in com/juul/kronicle (as that is convention for Java). Since Kotlin can have a flatter source code organization (whereas the empty initial package directories can be omitted), we have to instruct Codecov accordingly.

Codecov path fixes are structured as <files in coverage report>::<corresponding paths in project>; the fix added in this PR (com/juul/khronicle::) doesn't have a path on the right side of the ::, meaning that we are basically dropping the package directories that Kotlin doesn't require. For example, if we had a class in package com.juul.khronicle.example then the path fix would cause Codecov to look for the source file in example/.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.38%. Comparing base (dd278bd) to head (5d20f3d).

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #22       +/-   ##
===========================================
+ Coverage        0   94.38%   +94.38%     
- Complexity      0      120      +120     
===========================================
  Files           0       14       +14     
  Lines           0      178      +178     
  Branches        0       14       +14     
===========================================
+ Hits            0      168      +168     
- Misses          0        6        +6     
- Partials        0        4        +4     

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