feat(cli): Allow init to create any directory name

[nalanj]

• Allows any directory name. If no directory is given, init creates against the current directory, like git does.
• Uses .nango directory as a sentinel for determining if it's a nango integration folder or not
• Moves init into services directory for consistency
• Moves several of the files that were created in init with template strings to be files in templates, also for consistency
• Moves some tests around to be beside the init service
• Updates some docs

[nalanj]

@khaliqgant here's what I have so far on allowing init with any directory

[bodinsamuel]

Let's gooo