Implement common external function handlers in jerry-ext (#1787)

Added `handler` module to `jerry-ext` to contain implementation of
commonly used external function handlers: `assert`, `gc`, and
`print`.

Also adapted jerry-main to use jerry-ext/handler

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-main/CMakeLists.txt

@@ -58,7 +58,7 @@ endmacro()
 # Jerry standalones
 if(JERRY_CMDLINE)
   jerry_create_executable("jerry" "main-unix.c")
-  target_link_libraries("jerry" jerry-port-default)
+  target_link_libraries("jerry" jerry-ext jerry-port-default)
 endif()
 
 if(JERRY_CMDLINE_MINIMAL)