feat: add Kahn's algorithm for topological sort

[soondubu137]

Additions

Currently, there is only a DFS implementation for topological sort. Therefore, I'm adding Kahn's algorithm.

graph/kahn.go: an implementation of the Kahn's algorithm for topological sort, complemented with detailed comments. My code conforms to the Graph struct defined in graph/graph.go.
graph/kahn_test.go: 11 test cases.

Notes

1. I suggest renaming graph/topological.go to something else to indicate its DFS implementation.
2. graph/topological_test.go has a package-level variable called testCases. I think it's better to move it into the test function.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.65%. Comparing base (24c7f1f) to head (104beaf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #735      +/-   ##
==========================================
+ Coverage   87.59%   87.65%   +0.05%     
==========================================
  Files         208      209       +1     
  Lines        5524     5547      +23     
==========================================
+ Hits         4839     4862      +23     
  Misses        543      543              
  Partials      142      142              

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[soondubu137]

It seems the current codebase has some linting issue. I checked the errors reported by CI Lint, and I think they arise from existing files. (Same for PR #734)