fix: grouped operations and visitors by kind

Author: AndyOGo

src/operations/accessor/index.js

@@ -0,0 +1,3 @@
+export { default as variable } from './variable';
+export { default as missing } from './missing';
+export { default as missingSome } from './missingSome';

src/operations/missing.js renamed to src/operations/accessor/missing.js

@@ -0,0 +1,5 @@
+export { default as add } from './add';
+export { default as divide } from './divide';
+export { default as modulo } from './modulo';
+export { default as multiply } from './multiply';
+export { default as substract } from './substract';