[CLI] support adding cdk project to existing .net solution

From StackOverflow question, add support to add a CDK project to an existing solution.

Currently, cdk init will fail if run in a directory that already contains a solution (or any files):

❯ cdk init app --language=csharp
`cdk init` cannot be run in a non-empty directory!

Use Case

Add cdk project to an existing solution.

Proposed Solution

Remove check that current directory is empty. Probably add a check that files won’t be overwrittne.

Other

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

---

This is a 🚀 Feature Request

[shivlaks]

@ppittle that’s a good point. force is a little overloaded

I like the idea of --allow-non-empty. perhaps we could also make it a key we support in user/project settings (~/.cdk.json or cdk.json) so users would not have to keep specifying it every time.

[jhechtf]

Would be a great feature to have. It’s adding only a few specific directories, so I’m unsure why we’ve needed it to be done only on empty folders.