Convert `_json` to Argument Clinic

### Proposal:

As discussed in https://github.com/python/cpython/issues/137630#issuecomment-3191466847, for @hnnynh at the PyCon Korea sprints

cc @corona10 

### Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

### Links to previous discussion of this feature:

_No response_