Replace callargs_is_constructing with a method on CallArgs

[jdm]

I added it as a function in utils.rs for expediency, but it belongs in https://github.com/servo/mozjs/blob/b9edc816b6662a5986e190cdf53ae295049acb92/mozjs/src/jsimpls.rs#L221 instead.

[saito828koki]

I would like to work on this issue. Let me confirm what we would like to do before starting to work on it.

What we need to do is,

1. Add a new method is_constructing that is equivalent to callargs_is_constructing to JS::CallArgs https://github.com/servo/mozjs/blob/573f19426041ec6e1c166da71c0320257d1bd660/mozjs-sys/src/jsimpls.rs#L304
2. Replace all the usage of callargs_is_constructing in servo/servo like callargs_is_constructing(args) -> args.is_constructing()

Do I understand correctly?

[jdm]

Correct!

[saito828koki]

Thank you! Made a PR in mozjs repo.