Support reading default gems from $HOME/.default-gems file

[leomao10]

This is a proposal to make the support of default gems to be more consistent.

As current status:

• asdf-ruby using $HOME/.default-gems
• rvm-ruby using default/global gemsets in multiple locations:
  • ~/.rvm/gemsets/ree/1.8.7/p2010.02/global.gems
  • ~/.rvm/gemsets/ree/1.8.7/p2010.02/default.gems
  • ~/.rvm/gemsets/ree/1.8.7/global.gems
  • ~/.rvm/gemsets/ree/1.8.7/default.gems
  • ~/.rvm/gemsets/ree/global.gems
  • ~/.rvm/gemsets/ree/default.gems
  • ~/.rvm/gemsets/global.gems
  • ~/.rvm/gemsets/default.gems
• rbenv-default-gems using $(rbenv root)/default-gems

Just like .ruby-version, shall we have a consistent place for default gems setting?

[macarthy]

And support https://wiki.archlinux.org/title/XDG_Base_Directory so the default gems config chould be in ~/config/rbenv/default-gems (XDG_CONFIG_HOME). and the rbenv could install in XDG_DATA_HOME