Support private_class_method in the indexer

[kcdragon]

Motivation

Closes #2781

private_class_method can be used to declare class methods as private. Prior to this PR, this method was not setting the visibility to private for the methods in the arguments.

Implementation

My implementation is similar to the module_function implementation which was mentioned in the issue.
I handled the case where the arguments are Strings, Symbols or an Array of those. They are mentioned as the valid arguments in the Ruby docs.

Automated Tests

I added tests to cover private_class_method being passed a combination or Strings and Symbols or an Array of Strings and Symbols.

Manual Tests

In this example,

class Test
  def self.foo
  end

  def self.bar
  end

  private_class_method(:foo)
end

foo does not get suggested but bar does.

[kcdragon]

I have signed the CLA!

[andyw8]

Looks great!

Another way that private_class_method can be used is as a prefix to def, e.g.:

private_class_method def self.foo
  ...
end

Can we add a test to verify this doesn't cause anything to break?

[andyw8]

Suggested change

	assert_equal(Entry::Visibility::PRIVATE, entry.visibility)
	assert_predicate(entry.visibility, :private?)

(I updated some other occurrences in #2859)

[andyw8]

Suggested change

	assert_equal(Entry::Visibility::PRIVATE, entry.visibility)
	assert_predicate(entry.visibility, :private?)

[andyw8]

I pushed a commit to the branch below illustrating how an inline private_class_method def self.foo could be handled:

https://github.com/Shopify/ruby-lsp/tree/andyw8/def-private-class-method

[vinistock]

I think we actually don't support the inline visibility thing even for other operations, like

private def foo; end

I'm good with moving forward with this PR and addressing that in a separate change.

[andyw8]

@vinistock we do:

ruby-lsp/lib/ruby_indexer/lib/ruby_indexer/declaration_listener.rb

Lines 333 to 337 in 592495e

	# We want to restore the visibility stack when we leave a method definition with a visibility modifier
	# e.g. `private def foo; end`
	if node.arguments&.arguments&.first&.is_a?(Prism::DefNode)
	@visibility_stack.pop
	end

(and I verified manually)

[kcdragon]

Thanks for the feedback! I'll update the PR over the next couple of days.