Add new Rails/WhereNot cop

[fatkodima]

https://rails.rubystyle.guide/#where-not

# bad
User.where('name != ?', 'Gabe')
User.where('name != :name', name: 'Gabe')
User.where('name IS NOT NULL')
User.where('name NOT IN (?)', ['john', 'jane'])
User.where('name NOT IN (:names)', names: ['john', 'jane'])

# good
User.where.not(name: 'Gabe')
User.where.not(name: nil)
User.where.not(name: ['john', 'jane'])

I implemented in this PR the most basic and popular cases, to make this cop simpler and avoid manually parsing complex sql strings.

[andyw8]

I like the concept of checking for this, but I have a feeling that trying to parse SQL with regular expressions is going to be problematic.

[fatkodima]

Yes, definitely parsing complex SQL with regular expressions would not lead to something good, but in this case I'm parsing primitive and precise expressions (like column != ?, column NOT IN (?) and column IS NOT NULL), so I believe this is fine.

[fatkodima]

Updated.

[koic]

Thanks!