Refactor AstNodeExtensions - use properties with getters

[paul-dingemans]

Most likely we want to define this as property with a getter in AstNodeExtensions:

public val ASTNode.isLeaf: Boolean
   get() = firstChildNode == null

There are many functions defined in ASTNodeExtension which probably could better have been define in similar way. So we can also park this in a separate change.

Originally posted by @paul-dingemans in #2901 (comment)