Workflow improvements to stl init command

[bruce-hill]

The revised workflow for initializing a workspace looks like this:

• stl init and stl workspace init do the same thing
• If the user is not authenticated, prompt them to authenticate
• Prompt the user for each step, using Select choices when possible
• For choices like selecting which project to use, the user gets a select menu with one of the options being "New Project", which lets them type in a project name.
• The default configuration options go into .stainless/project.json, .stainless/stainless.yml, .stainless/openapi.json, but if there is a local file matching any of those names, it's suggested as the option
• Once everything is configured, we will download your stainless.yml and OpenAPI spec and put them in the places you specified (and confirm overwriting if a file exists there already).
• If any parameters are specified by CLI flags, we don't prompt you for them.

[bruce-hill]

@yjp20 Bumping this review request

[yjp20]

honestly haven't had time to play with the CLI much, but the changes looks good to me