Improve member count.

[M-Yamashita01]

The action counts github_membership and github_repository_collaborator in the tf file as members.
However, If a user starts writing the GitHub account management in a different way in the tf file, the action will not be counted correctly.
Therefore, I have improved the action to count members from terraform plan results.

[codecov-commenter]

Codecov Report

Merging #45 (8e0709b) into main (1077d0e) will decrease coverage by 0.38%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
- Coverage   96.80%   96.42%   -0.38%     
==========================================
  Files          10        8       -2     
  Lines         250      224      -26     
==========================================
- Hits          242      216      -26     
  Misses          8        8              

Flag	Coverage Δ
unittests	96.42% <100.00%> (-0.38%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/check_members_action.rb	87.23% <100.00%> (ø)
lib/config_validator.rb	91.30% <100.00%> (ø)
lib/terraform_reader.rb	100.00% <100.00%> (ø)
spec/config_validator_spec.rb	100.00% <100.00%> (ø)
spec/terraform_reader_spec.rb	100.00% <100.00%> (ø)

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