Improve analyzer "Use exception filter" (RCS1236)

This
```csharp
try
{
return Load(path);
}
catch (IOException)
{
}
catch (UnauthorizedAccessException)
{
}
catch (XmlException)
{
}
```
can be replaced with this
```csharp
try
{
return Load(path);
}
catch (Exception ex) when (ex is IOException
|| ex is UnauthorizedAccessException
|| ex is XmlException)
{
}
```

Start by locating the RCS1236 analyzer entry point and its existing tests. Use the try/catch example as the acceptance case: the analyzer should recognize consecutive catches for IOException, UnauthorizedAccessException, and XmlException and suggest the shown exception-filter form.

Written by the indexing model from the issue text.