Add reference to function string literal parameter for ContextMenuItemAttribute

The [`ContextMenuItemAttribute`](https://docs.unity3d.com/ScriptReference/ContextMenuItemAttribute.html) type includes a `function` parameter that is a string literal referencing a function name on the current type.

Add a reference to the method:

* Code completion
* Find usages
* Rename
* Inspections for method signature
* Create from usage
* Suggest using `nameof`

Start by locating the Unity support for ContextMenuItemAttribute and how string literals are resolved to methods. Review the requested behaviors—completion, usages, rename, signature inspections, create-from-usage, and nameof suggestions—and verify each one against a representative attribute usage. Done means the function parameter is treated as a reference to a method on the current type across those IDE features.

Written by the indexing model from the issue text.