feat: 增加flatbuffer的variables字段

Author: heiazu

__test__/fixure/style.bin

@@ -81,13 +81,19 @@ table PseudoKey {
   is_int:bool;
 }
 
+table KeyValueString {
+  key:string;
+  value:string;
+}
+
 table Style {
   declarations:[DeclarationTuple];
   media:uint8;
   pseudo:uint8;
   pseudo_key:[PseudoKey];
   pseudo_val:string;
   selector:[Selector];
+  variables:[KeyValueString];
 }
 
 table Font {