Satisfy trait type condition

[filchristou]

is_directed(::Type{CustomGraph})::Bool should be defined to be consistent with Graphs.jl
https://juliagraphs.org/Graphs.jl/dev/ecosystem/interface/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.95%. Comparing base (f7553c7) to head (2537441).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   92.46%   91.95%   -0.51%     
==========================================
  Files           5        6       +1     
  Lines         345      348       +3     
==========================================
+ Hits          319      320       +1     
- Misses         26       28       +2     

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[gdalle]

This is indeed a bug, I just caught it with https://github.com/gdalle/GraphsInterfaceChecker.jl

[oameye]

I am also hitting this