Change default completor from regexp to type-completor

[tompng]

IRB will use type completor as default. If repl_type_completor is not available (not installed or not added to Gemfile), fallbacks to RegexpCompletor.

When type completor is not available, IRB shows warning message only If --type-completor or IRB.conf[:COMPLETOR] = :type is explicitly specified.

$ irb -f
irb(main):001> irb_info
(TypeCompletor)

$ echo "gem 'irb', path: 'path/to/irb'" > Gemfile

$ bundle exec irb -f
(no waarning)
irb(main):001> irb_info
(RegexpCompletor)

$ bundle exec irb -f --type-completor
TypeCompletor requires `gem repl_type_completor`: cannot load such file -- repl_type_completor
irb(main):001> irb_info
(RegexpCompletor)

[st0012]

Since repl_type_completor only has ~64k downloads atm, this means almost all IRB sessions after this change will trigger a LoadError unnecessarily. And at the same time, this change isn't actually making more people use repl_type_completor. It simply saves the existing users the need to configure it.

IMO the right solution here is to propose making repl_type_completor a bundled gem. And only until that happens, we make it a default for IRB with a Ruby version check as older Ruby versions still won't have it installed by default.