Complete writer methods with self receiver when receiver is self

[antw]

Motivation

Closes #3528

Setter methods where the receiver is self were being completed without the explicit self. prefix. This would result in local variable assignment rather than calling the writer method.

For example, when typing b in a class method and expecting completion for baz=, the LSP would suggest:

def bar
  baz= # This assigns to a local variable, not calling the writer method
end

Instead of the correct:

def bar
  self.baz=
end

Implementation

• Modified the add_method_completions method in completion.rb to detect when completing setter methods with a self receiver.
• The changes only affect files where Sorbet type checking is not enabled; those use Sorbet's completion, which has the same issue, but is outside the scope of Ruby LSP (as far as I know).

The implementation checks node.receiver.nil? to detect implicit self and avoid adding self. when the user has already typed the receiver.

Note

This is my first ruby-lsp PR. Feedback on the approach would be appreciated, particularly:

• I read through the documentation for CompletionItem and it feels like setting the newText is the correct option (leaving the label and filterText as before) but I'd be curious if reviewers feel otherwise.
• I put spaces around the "=" as suggested in the issue since that's how most people would write this..

Automated Tests

Added test coverage in completion_test.rb with three scenarios:

• Implicit self receiver (b in class method) - verifies self. prefix is added to completion text
• Explicit self receiver (self.b in class method) - verifies no additional self. prefix is added
• External receiver (foo.b outside class) - verifies no self. prefix is added

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