Add JERRY_FEATURE_DEBUGGER (#1738)

Until now, it was not possible to query the engine whether its
build configuration contained debugging support. This commit adds
the `JERRY_FEATURE_DEBUGGER` label to `jerry_feature_t` and extends
`jerry_is_feature_enabled` to support the new feature flag.
Additionally, the command line tool `jerry` was enhanced to report
a warning when invoked with `--start-debug-server` but linked to a
non-debuggable engine.

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

jerry-core/jerryscript.h

@@ -85,6 +85,7 @@ typedef enum
   JERRY_FEATURE_REGEXP_DUMP, /**< regexp byte-code dumps */
   JERRY_FEATURE_SNAPSHOT_SAVE, /**< saving snapshot files */
   JERRY_FEATURE_SNAPSHOT_EXEC, /**< executing snapshot files */
+  JERRY_FEATURE_DEBUGGER, /**< debugging */
   JERRY_FEATURE__COUNT /**< number of features. NOTE: must be at the end of the list */
 } jerry_feature_t;