Add promise-function-async rule

https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/promise-function-async.md

Author: sindresorhus

index.js

@@ -180,6 +180,7 @@ module.exports = {
 		// '@typescript-eslint/prefer-interface': 'error',
 
 		'@typescript-eslint/prefer-namespace-keyword': 'error',
+		'@typescript-eslint/promise-function-async': 'error',
 		'@typescript-eslint/restrict-plus-operands': 'error',
 		'@typescript-eslint/type-annotation-spacing': 'error'
 	}