Fix TS type for resolveConfig function

Summary:
Related: react-native-community/cli#1889.

Changelog: **[Types]** Fix return type on `'metro-config'.resolveConfig`

Reviewed By: motiz88

Differential Revision: D44540075

fbshipit-source-id: 182c3d7ac3c75446a905cb2988bef8044dea2a76

Author: huntie

packages/metro-config/types/loadConfig.d.ts

@@ -27,7 +27,7 @@ export function loadConfig(
 export function resolveConfig(
   filePath?: string,
   cwd?: string,
-): Promise<ConfigT>;
+): Promise<CosmiConfigResult>;
 
 export function mergeConfig(
   defaultConfig: InputConfigT,