Added hhast-migrate --add-xhp-children-declaration-method

This uses the new child validation approach supported in xhp-lib v3.1. For example, it replaces:

class :test:nested-rule extends :x:element {
  children (:div | (:code+));
}

... with this:

class :test:nested-rule extends :x:element {
  use XHPChildDeclarationConsistencyValidation;

  children (:div | (:code+));

  // this is equivalent to the previous declaration
  protected static function getChildrenDeclaration(): XHPChild\Constraint {
    return XHPChild\anyOf(
      XHPChild\ofType<:div>(),
      XHPChild\atLeastOneOf(XHPChild\ofType<:code>()),
    );
  }

The trait adds a runtime check that the old and new declaration are consistent

Add hhast-migrate --remove-xhp-child-declarations

This removes children declarations, and if an XHPChildDeclarationConsistencyValidation trait is present, replaces it with XHPChildValidation

hhast-migrate --add-xhp-children-declaration-method should generally be used first to add the new declaration, and allow runtime testing to ensure the new version is equivalent.

Add hhast-migrate --demangle-xhp-class-names

This replaces - in XHP class names in _; this is not yet required - is is preparatory work for XHP namespace support.