Compare two JSON files and show semantic differences.
```console
$ diffx tests/fixtures/file1.json tests/fixtures/file2.json
? 1
~ age: 30 -> 31
~ city: "New York" -> "Boston"
+
```
When files are identical, no output is shown and exit code is 0.
```console
$ diffx tests/fixtures/file1.json tests/fixtures/file1.json
```