Add default implementation for is_dir()

[lieryan]

Description

is_dir() is the name used by os.PathLike interface, while is_folder() was the original name used by rope's Resource.

At some point, Resource became an implementor of os.PathLike but it never provided an implementation for is_dir().

Checklist (delete if not relevant):

• I have added tests that prove my fix is effective or that my feature works
• I have updated CHANGELOG.md

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.29%. Comparing base (d2c5127) to head (392f729).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #870   +/-   ##
=======================================
  Coverage   95.28%   95.29%           
=======================================
  Files         134      134           
  Lines       26534    26541    +7     
=======================================
+ Hits        25284    25291    +7     
  Misses       1250     1250           

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