The `ReferenceInfo.index` is always missing from the completion provider

[raylras]

Langium version: 4.0.0
Package name: langium

Related discussion: #2025

Steps To Reproduce

• Not applicable

Link to code example:

The ReferenceInfo interface has an index property:

langium/packages/langium/src/syntax-tree.ts

Lines 137 to 146 in 38d5f16

	/**
	* Information about a cross-reference. This is used when traversing references in an AST or to describe
	* unresolved references.
	*/
	export interface ReferenceInfo {
	reference: Reference | MultiReference
	container: AstNode
	property: string
	index?: number
	}

However, the completion provider always misses it:

langium/packages/langium/src/lsp/completion/completion-provider.ts

Lines 444 to 448 in 38d5f16

	const refInfo: ReferenceInfo = {
	reference,
	container: node,
	property: assignment.feature
	};

The current behavior

The refInfo is missing the index in any case.

The expected behavior

The index should be set if available.