Add support for endless methods to Style/SingleLineMethods.

[dvandersluis]

Style/SingleLineMethods would raise an error when encountering a Ruby 3.0 endless method. I updated it to ignore endless methods (Style/EndlessMethods in #9281 handles them instead).

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
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• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• 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.

[dvandersluis]

documentation_syntax_check was failing on the ruby 3.0 syntax in examples, so I've updated the base parser it uses to be Parser::Ruby30 instead of Parser::Ruby27.

[bbatsov]

Shouldn't #single_line? return true on this case as well?

[bbatsov]

My bad. I just realized the cop treats specially empty definitions and now endless definition. I guess we need to improve its description a bit.

[dvandersluis]

@bbatsov just realized my PR description was a bit wonky so I've fixed that.

single_line? likely returns true for an endless method (since they generally should be on a single line I believe), however, a single line method as far as this cop is concerned is one that is of the form def x; something end, and the correction code was failing because node.loc.end doesn't exist for an endless method.

[bbatsov]

Yeah, I figured this out with a bit of a delay. 😆

[bbatsov]

I'll merge this, even though it seems something's wrong with the single_line? method.