core: CfnResource.isCfnResource is too permissive

### Describe the bug

I created a construct class with a cfnResourceType property. I use that type to search for a CfnResource under the parent of that construct. But my non-CfnResource class is recognized as a CfnResource by the CDK because it has a cfnResourceType property! Wrong!

### Expected Behavior

```
public static isCfnResource(x: any): x is CfnResource {
return CfnElement.isCfnElement(x) && x.cfnResourceType !== undefined;
}
```

That would be a lot better because isCfnElement has an RTTI symbol property.

### Current Behavior

```
public static isCfnResource(x: any): x is CfnResource {
return x !== null && typeof(x) === 'object' && x.cfnResourceType !== undefined;
}
```

### Reproduction Steps

```
class BucketHelperClass extends Construct {
readonly cfnResourceType = CfnBucket.CFN_RESOURCE_TYPE_NAME
}
let helper = new BucketHelperClass(new App(), "Helper")
if (CfnResource.isCfnResource(helper)) {
throw new Error('No it is not a CfnResource!');
}
```

### Possible Solution

Either this:
```
public static isCfnResource(x: any): x is CfnResource {
return CfnElement.isCfnElement(x) && x.cfnResourceType !== undefined;
}
```

Or maybe:

```
const CFN_RESOURCE_SYMBOL = Symbol.for('@aws-cdk/core.CfnResource');
```

### Additional Information/Context

When I try to synthesize a stack with a fake CfnResource, I get the following error:

```
[CDK] TypeError: source.addDependency is not a function
```

### CDK CLI Version

2.120.0 (build 58b90c4)

### Framework Version

_No response_

### Node.js Version

10.2.5

### OS

MacOS

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Locate the CfnResource.isCfnResource implementation and inspect how CfnElement.isCfnElement identifies constructs. Reproduce the issue with the BucketHelperClass example, then verify that a construct exposing cfnResourceType is not treated as a CfnResource while genuine CfnResources still are.

Written by the indexing model from the issue text.