Update LS for natural expression changes

[MaryamZi]

Purpose

$title.

[Copilot]

Pull Request Overview

This PR updates the language server (LS) to support the new "natural" expression changes in the Ballerina language. The changes add comprehensive completion support for the natural type across all language contexts.

• Adds natural type completion as a basic type suggestion
• Includes natural (..) {..} snippet completion for natural expressions with parameter and body placeholders
• Updates test configuration files to include expected completion items for the natural type

Comments suppressed due to low confidence (1)

langserver-core/src/test/resources/completion/expression_context/config/typeof_expression_ctx_config5.json:474

• The removal of the 'code' completion item suggests a behavior change that should be covered by tests. Ensure there are corresponding test cases that verify the 'code' completion is no longer available in this context.

    {