Add method hasChildren to DetailAST

[pbludov]

From sevntu-checkstyle/sevntu.checkstyle#794 (comment)

The most common usage of the method getChildCount is to compare the count to zero. For example. This is not efficient nor readable. Both issues can be fixed by adding a new method to the DetailAST interface:

default boolean hasChildren() {
    return getFirstChild() != null;
}

[romani]

I am ok to add this methood.
@rnveach , please review and put label.

[rnveach]

@romani I was ok with it until I saw the default method in the interface. I personally would just prefer to see the implementation in the class. I am old school.

[romani]

ok, lets make method in interface as declaration.

[pbludov]

Started hcoles/pitest#725 for the Pitest issue.

[romani]

Fix is merged