Introduce new jerry-ext methods to ease property registration (#2715)

New methods
* jerryx_set_properties: Allows multiple property registration for via a single call
* jerryx_set_property_str: Allows property registration without the need to
    create the property name JS value. The property name can be directly passed
    as a `const char*` value and must be zero terminated.
* jerryx_get_property_str: Allows getting a property value on a given object.
    The property name can be directly passed as a `const char*` value.
* jerryx_has_property_str: Allows checking if a property exists on a given object.

JerryScript-DCO-1.0-Signed-off-by: Peter Gal pgal.u-szeged@partner.samsung.com

Author: galpeter