Improve install script to warn users with existing installations

[jaydrogers]

👉 Describe the problem

If a user already has a ~/.spin directory, the installation hangs.

👥 Problem evidence & reach

Related issue: #52 (reported by @danmatthews)

🥰 Describe the "impact" on users?

Some users may get confused.

🏆 How to solve this problem

• Detect if ~/.spin exists
• If so, display a warning and give them instructions on how to resolve

💯 How do we validate the problem is solved?

A user should be able to complete the following steps and not get confused:

1. Install spin
2. Attempt to install spin again