Detect resource leaks

Hi! First of all, thanks for this library 👍🏾.

So... I use Jimfs in my tests and I reckon that's what many people do. I think it'd be fruitful if Jimfs could detect open resources that aren't released properly (by resources I mean anything created by the FS that's implementing `Closeable`, including channels, directory streams, etc...). This makes it really easy to ensure tested components properly close file resources. I realize it'll be silly if this suddenly became the default behavior, so I'd expect this to be explicitly enabled in the FS's `Configuration`. It'd be preferable if this could be checked in `FileSystem::close`, so the following pattern can be utilized (depending on the testing framework):

```java
private FileSystem fs;

@BeforeEach
void setUp() {
var config = Configuration.builder(PathType.unix()).setDetectLeaksOnClosure(true).build();
fs = Jimfs.newFileSystem(config);
}

@AfterEach
void tearDown() throws IOException {
if (fs != null) {
fs.close(); // Throws in case of resource leaks
}
}
...
```
I notice resource registration is handled by the `FileSystemState` class. Currently, it seems to only close registered resources. I think most behavior could be implemented there, but you guys know better. The class seems to receive possibly custom `Closeable` instances, so it may need to check the classes of registered resources before complaining. Perhaps, specific resource types could explicitly opt-in for the checks by the configuration (e.g. only check FileChannels).

Thanks in advance!