Should init() infer, or prompt, what the default snapshot type should be?

[kevinushey]

If a user calls renv::init() on a project containing a DESCRIPTION file, should we:

• Assume they want to use "explicit" snapshots in the project, and set that project setting for them?
• Or, prompt them to do so?
• Or, do nothing?

[hadley]

I like the idea of prompting, or at least messaging something like:

* You project has a `DESCRIPTION` but renv will capture all dependenices
* Use `settings$snapshot.type("explict")` to only capture dependencies defined in the `DESCRIPTION`

[kevinushey]

Do we want different behavior for R packages versus "regular" projects that have a DESCRIPTION file?