Add a --dry-run option

[mattxwang]

What is the problem you're trying to solve?

Most tools that perform destructive actions have a --dry-run option, where the tool prints out everything it would have done normally, but doesn't actually touch the filesystem (or perform other actions). This can be helpful to validate how the tool would work before committing to it, or debugging.

In implementing #1, I tabled this feature for a follow-up.

What solution would you like to see?

A --dry-run option for this tool.

Since this would be one of our first options, we can decide what our approach is for argument parsing; I've seen yargs used frequently, including in create-astro. That being said, I'm not wedded to any implementation.

I'm happy to contribute this feature, especially as it's a follow-up to #1.