Add new Lint/LiteralAssignmentInCondition cop

[koic]

Summary

This PR adds a new Lint/LiteralAssignmentInCondition cop, which checks for literal assignments in the conditions of if, while, and until. It emulates the following Ruby warning:

$ ruby -we 'if x = true; end'
-e:1: warning: found `= literal' in conditional, should be ==

As a lint cop, it cannot be determined if == is appropriate as intended, therefore this cop does not provide autocorrection.

# bad
if x = 42
  do_something
end

# good
if x == 42
  do_something
end

# good
if x = y
  do_something
end

Literal assignment used within a condition always triggers a Ruby warning, regardless of parentheses. This PR prevents the Ruby warning from being overlooked due to the ruby -w option.

Additional Information

Here, the differences between similar cops, Lint/LiteralAssignmentInCondition cop and Lint/LiteralAsCondition cop are discussed.

Lint/LiteralAssignmentInCondition cop

A similar Lint/LiteralAssignmentInCondition cop already exists, but as mentioned above, literal assignments are Ruby's warned about regardless of the presence of parentheses:

$ ruby -we 'if (x = 42); end'
-e:1: warning: found `= literal' in conditional, should be ==

Therefore, a new cop has been created separate from Lint/LiteralAssignmentInCondition.

Furthermore, as documented for Lint/LiteralAssignmentInCondition cop, literal assignments are never considered good case, so = true has been replaced with = value.

Lint/LiteralAsCondition cop

Additionally, the existing Lint/LiteralAsCondition cop addresses a slightly different Ruby warning:

$ ruby -we 'if 42; end'
-e:1: warning: literal in condition

This is another reason for choosing to implement a new cop.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Looks good to me. Thanks!