Re-export metro-core's Terminal from metro

Summary:
Following D44870484 with the goal of decoupling RN CLI from an exact Metro version - `react-native-community/cli` wraps `Terminal` for its `reporter` implementation when starting Metro server, here:

https://github.com/react-native-community/cli/blob/v12.0.0-alpha.2/packages/cli-plugin-metro/src/commands/start/runServer.ts#L45-L47

This is the only dependency `cli-plugin-metro` has on `metro-core` - by re-exporting it from `metro` we can [remove a dependency](https://github.com/react-native-community/cli/blob/v12.0.0-alpha.2/packages/cli-plugin-metro/package.json#L16) from the CLI.

Changelog:
```
* **[Feature]** Re-export `metro-config`'s `resolveConfig` from `metro`.
```

Reviewed By: hoxyq

Differential Revision: D44870838

fbshipit-source-id: 94baf444f495439e45ff77340601afd6900bbc3f

packages/metro/src/index.flow.js

@@ -44,6 +44,7 @@ const {
   mergeConfig,
   resolveConfig,
 } = require('metro-config');
+const {Terminal} = require('metro-core');
 const {InspectorProxy} = require('metro-inspector-proxy');
 const net = require('net');
 const {parse} = require('url');
@@ -126,6 +127,8 @@ export type RunBuildOptions = {
 type BuildCommandOptions = {} | null;
 type ServeCommandOptions = {} | null;
 
+exports.Terminal = Terminal;
+
 export type {MetroConfig};
 
 async function getConfig(config: InputConfigT): Promise<ConfigT> {

packages/metro/types/index.d.ts

@@ -30,6 +30,7 @@ import type {default as MetroServer, ServerOptions} from './Server';
 import type {OutputOptions, RequestOptions} from './shared/types';
 
 export {loadConfig, mergeConfig, resolveConfig} from 'metro-config';
+export {Terminal} from 'metro-core';
 
 export {HttpServer, HttpsServer};