* doc/*: Move entry to default gems category from standard library.
  * lib/shell/*: Added `Shell::VERSION` and re-used it with @RELEASE_VERSION.
  * test/shell/test_command_processor.rb: added missing require for test library.
  * tool/sync_default_gems.rb: Support shell library. We need to ignore shellwords.rb when
    syncing shell* files.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64004 b2dd03c8-39d4-4d8f-98ff-823fe69b080e