Add warning and confirmation before running theme doctor

[FKauwe]

WHY are these changes introduced?

Fixes #1016

Building off the architecture for a new theme doctor command in this PR, I introduce a warning and confirm with the user before executing the theme doctor test suite, as the tests will interact with user's remote shop.

WHAT is this pull request doing?

• add the renderConfirmationPrompt function to the doctor/theme/index.ts file
• add a new unittest file and 2 unittests to verify that behavior for User Confirms and User Cancels are correct

looks like this when User Confirms(test suite executes):

and when User Cancels (exit the command):

How to test your changes?

pull my branch down locally
p build
run shopify-dev doctor theme -e <local-env-name> in external terminal
see the warning and prompt and the expected behaviors between Yes/No