How to check which packages are available by default in the used Emacs version

[akirak]

Problem: While twist allows the user to override built-in packages (e.g. by adding :ensure t to corresponding use-package declarations), newer versions of Emacs come with more packages that had been once only available from ELPA (e.g. transient, eglot, etc.). It would be better to somehow let the user know which packages have become shipped with the Emacs version used in his/her own config.

See #112 (comment).

[akirak]

#115 is a solution to this problem. The user needs to run the command.