Add analyzer for checking for null parameters on assignation

It would be nice if you had an analyzer that would e.g. change

```cs
private string Arg1;
private Stream Arg2;

public void Foo(string arg1, Stream arg2)
{
this.Arg1 = arg1;
this.Arg2 = arg2;
}
```

to

```cs
private string Arg1;
private Stream Arg2;

public void Foo(string arg1, Stream arg2)
{
this.Arg1 = arg1 ?? throw new ArgumentNullException(nameof(arg1));
this.Arg2 = arg2 ?? throw new ArgumentNullException(nameof(arg2));
}
```

I think this should only be available when the line is a simple assignation (`x = param`).

The issue provides no repository files or tests to start from. Locate existing C# analyzer and code-fix entry points and their tests, then verify how simple parameter-to-member assignments are diagnosed. Done means the analyzer offers null checks for those assignments while leaving other assignment forms unchanged.

Written by the indexing model from the issue text.