Correct some jest types (#48923)

# What?

This PR just refines some of the typescript types around `nextJest`, specifically refining the argument `customJestConfig` and the return value.

### Why?

Before this PR, the return type of `nextJest` was inferred by typescript as `any` which is inaccurate and would require type casting for typescript to enable it to be used in a jest configuration written in typescript.


I didn't add an issue for this since it was so trivial, but I can do so if it would be helpful.

packages/next/src/build/jest/jest.ts

@@ -8,6 +8,7 @@ import * as Log from '../output/log'
 import { findPagesDir } from '../../lib/find-pages-dir'
 import { loadBindings, lockfilePatchPromise } from '../swc'
 import type { JestTransformerConfig } from '../swc/jest-transformer'
+import type { Config } from '@jest/types'
 
 async function getConfig(dir: string) {
   const conf = await loadConfig(PHASE_TEST, dir)
@@ -56,10 +57,14 @@ module.exports = createJestConfig(customJestConfig)
 */
 export default function nextJest(options: { dir?: string } = {}) {
   // createJestConfig
-  return (customJestConfig?: any) => {
+  return (
+    customJestConfig?:
+      | Config.InitialProjectOptions
+      | (() => Promise<Config.InitialProjectOptions>)
+  ) => {
     // Function that is provided as the module.exports of jest.config.js
     // Will be called and awaited by Jest
-    return async () => {
+    return async (): Promise<Config.InitialProjectOptions> => {
       let nextConfig
       let jsConfig
       let resolvedBaseUrl

packages/next/src/build/swc/jest-transformer.ts

@@ -37,7 +37,8 @@ import type {
 import type { Config } from '@jest/types'
 import type { NextConfig, ExperimentalConfig } from '../../server/config-shared'
 
-export interface JestTransformerConfig {
+type TransformerConfig = Config.TransformerConfig[1]
+export interface JestTransformerConfig extends TransformerConfig {
   jsConfig: any
   resolvedBaseUrl?: string
   pagesDir?: string