Ruby » Ruby master

Merge bundler to standard libraries.

rubygems 2.7.x depends bundler-1.15.x. This is preparation for
rubygems and bundler migration.

• lib/bundler.rb, lib/bundler/*: files of bundler-1.15.4
• spec/bundler/*: rspec examples of bundler-1.15.4. I applied patches.
  • https://github.com/bundler/bundler/pull/6007
  • Exclude not working examples on ruby repository.
  • Fake ruby interpriter instead of installed ruby.
• Makefile.in: Added test task named test-bundler. This task is only
  working macOS/linux yet. I'm going to support Windows environment later.
• tool/sync_default_gems.rb: Added sync task for bundler.

[Feature #12733][ruby-core:77172]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59779 b2dd03c8-39d4-4d8f-98ff-823fe69b080e