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/jerry.c

@@ -618,6 +618,9 @@ bool jerry_is_feature_enabled (const jerry_feature_t feature)
 #ifdef JERRY_ENABLE_SNAPSHOT_EXEC
           || feature == JERRY_FEATURE_SNAPSHOT_EXEC
 #endif /* JERRY_ENABLE_SNAPSHOT_EXEC */
+#ifdef JERRY_DEBUGGER
+          || feature == JERRY_FEATURE_DEBUGGER
+#endif /* JERRY_DEBUGGER */
           );
 } /* jerry_is_feature_enabled */