Add new Style/CaseLikeIf cop

[fatkodima]

This cop tries to convert if-elsifs to case-whens.

Algorithm, in brief:

case target
when conditions1
  # ...statements...
when conditions2
  # ...statements...
else
  # ...statements...
end

1. Find possible target from if condition
2. if found, then for each branch collect conditions and check if they are all depend on target and can be converted to case
3. if all branches are convertible to cases, convert them

Closes #7736

[bbatsov]

Great work! I wonder if we can do better with the name, though. I'm thinking of CaseLikeIf (case-like if) or SingleValueDispatchIf, but maybe someone from @rubocop-hq/rubocop-core will have better ideas.

[koic]

CaseLikeIf (case-like if)

It sounds very comfortable to me :-)

[bbatsov]

The changes look good, but your branch has to be rebased on top of the current master branch due to merge conflicts.

[fatkodima]

Rebased.