Try to fix rewrite for self file extension method [Workaround]

Hi, let's try to make a rewriter for extension methods. It would allow to avoid ambiguous call compilation errors :

```c#
public static class SomeClass {

public static void MyExtension(this string myString){ ... }

public static void MyMethod(){ "someValue".MyExtension() } //Will cause an ambiguous call in the compiler

public static void MyMethod(){ MyExtension("someValue") } //This works and is the current workaround.
}
```

The issue provides no file, test, or entry point to begin from; locate the rewriter responsible for extension-method calls and study how the current workaround is handled. Done means the rewriter supports the shown self-file extension call without the compiler's ambiguous-call error, while preserving the existing direct-call behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.