Idea: Make submodules: true the default for Bundler 3

[ankane]

Hi, I wanted to get thoughts on making submodules: true the default for Bundler 3 for git (and related) sources.

Pro: Repos with submodules should just work.
Con: May unnecessarily download unneeded submodules.

[deivid-rodriguez]

In my opinion, this makes sense as a better default for Bundler 3 👍.