is.EqualJSON support

Hi,

It would be helpful to have a is.EqualJSON(a, b string) function to compare JSON values structurally, ignoring formatting and key order.

Example:

expected := `{"name":"Alice","age":30}`
actual := `{
    "age": 30,
    "name": "Alice"
}`

is.EqualJSON(expected, actual)

Internally, this could unmarshal both inputs into interface{} and then use is.Equal.

If you're open to it, I’d be happy to contribute a PR.

Thanks!