make private properties use leading underscore

index.js

@@ -36,7 +36,13 @@ module.exports = {
     '@typescript-eslint/camelcase': 'off',
     '@typescript-eslint/naming-convention': ['error',
       { selector: 'default', format: ['snake_case'] },
-      { selector: 'typeLike', format: ['PascalCase'] }
+      { selector: 'typeLike', format: ['PascalCase'] },
+      {
+        selector: 'property',
+        modifiers: ['private'],
+        format: ['snake_case'],
+        leadingUnderscore: 'require'
+      }
     ],
     'id-match': ['error', '^([a-z][a-z0-9_]*|[A-Z][a-zA-Z]*)$', { onlyDeclarations: true }]
   }