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:

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

is.EqualJSON(expected, actual)
```

## Possible implementation idea
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!

Start by locating the existing is.Equal implementation and related tests in the repository. Use the issue's example and proposed JSON unmarshalling approach as the acceptance criteria: equivalent JSON should compare equal despite formatting or key order, and the test suite should cover this behavior.

Written by the indexing model from the issue text.