Print environment paths in project command

[soutaro]

steep project prints the directories to load library RBSs from for troubleshooting.

Example:

$ bundle exec steep project
Target:
  app:
    sources:
      patterns:
        - lib
      ignores:
      files:
        - lib/conference.rb
    signatures:
      patterns:
        - sig
      files:
        - sig/conference.rbs
    libraries:
      - set
      - pathname
    library dirs:
      - core: /Users/soutaro/src/steep/vendor/bundle/ruby/2.7.0/gems/rbs-0.16.0/core
      - set: /Users/soutaro/src/steep/vendor/bundle/ruby/2.7.0/gems/rbs-0.16.0/stdlib/set/0
      - pathname: /Users/soutaro/src/steep/vendor/bundle/ruby/2.7.0/gems/rbs-0.16.0/stdlib/pathname/0