Add new Style/RaiseString cop

[tjwp]

This cop checks if raise or fail is called with just a String
and suggests using an exception class.

This cop is disabled by default.

This change partly addresses #3066. There are some projects where I've wanted all raise calls to use custom errors and this would help to enforce that.

• Wrote [good commit messages][1].
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it)
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

[jonas054]

A very similar cop, Style/ImplicitRuntimeError, is added in #3182.

[alexdowad]

A couple good things here: both raise and fail are caught, and with both str and dstr arguments. I neglected to handle a couple of those cases.

[jonas054]

So if you could rewrite this as an improvement of Style/ImplicitRuntimeError, which has been merged to master, that would be great, @tjwp.

[tjwp]

I'm going to close this and will work on a separate PR with improvements for Style/ImplicitRuntimeError.

[tjwp]

Added improvements here: #3225