Fix an incorrect auto-correct for Style/SingleLineMethods

[koic]

This PR fixes an incorrect auto-correct for Style/SingleLineMethods when using return, break, or next for one line method body in Ruby 3.0.

These are not supported in the endless method definition.

% ruby -v
ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-darwin19]

% cat example.rb
def foo(argument) break bar(argument); end
def foo(argument) next bar(argument); end
def foo(argument) return bar(argument); end

% ruby -c example.rb
Syntax OK

% bundle exec rubocop --only Style/SingleLineMethods -a
(snip)

Inspecting 1 file
C

Offenses:

example.rb:1:1: C: [Corrected] Style/SingleLineMethods: Avoid
single-line method definitions.
def foo(argument) break bar(argument); end
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
example.rb:2:1: C: [Corrected] Style/SingleLineMethods: Avoid
single-line method definitions.
def foo(argument) next bar(argument); end
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
example.rb:3:1: C: [Corrected] Style/SingleLineMethods: Avoid
single-line method definitions.
def foo(argument) return bar(argument); end
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 3 offenses detected, 3 offenses corrected

% cat example.rb
def foo(argument) = break bar(argument)
def foo(argument) = next bar(argument)
def foo(argument) = return bar(argument)

% ruby -ce 'def foo(argument) = break bar(argument)'
-e:1: syntax error, unexpected local variable or method, expecting
end-of-input
def foo(argument) = break bar(argument)

% ruby -ce 'def foo(argument) = next bar(argument)'
-e:1: syntax error, unexpected local variable or method, expecting
end-of-input
def foo(argument) = next bar(argument)

% ruby -ce 'def foo(argument) = return bar(argument)'
-e:1: syntax error, unexpected local variable or method, expecting
end-of-input
def foo(argument) = return bar(argument)

---

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]

Interesting edge case! How did you even find this?

[koic]

I found it while investigating standardrb/standard#309. There may be several other problems with this issue, so I'm looking at them in order.