Added constants for feature importance [skip ci]

Author: ankane

lib/lightgbm/booster.rb

@@ -65,9 +65,9 @@ def feature_importance(iteration: nil, importance_type: "split")
       importance_type =
         case importance_type
         when "split"
-          0
+          FFI::C_API_FEATURE_IMPORTANCE_SPLIT
         when "gain"
-          1
+          FFI::C_API_FEATURE_IMPORTANCE_GAIN
         else
           -1
         end

lib/lightgbm/ffi.rb

@@ -20,6 +20,9 @@ module FFI
     C_API_PREDICT_LEAF_INDEX = 2
     C_API_PREDICT_CONTRIB = 3
 
+    C_API_FEATURE_IMPORTANCE_SPLIT = 0
+    C_API_FEATURE_IMPORTANCE_GAIN = 1
+
     # error
     attach_function :LGBM_GetLastError, %i[], :string