fix: fixes importMap type in monorepo usage (#15203)

### What?

Adds a `/** @type import("payload").ImportMap */` annotation to the
generated importMap.

### Why?

In a monorepo, it's common to use [Project
References](https://www.typescriptlang.org/docs/handbook/project-references.html)
so that Typescript can do dependency analysis on the packages in your
project. Often, projects will have a single root `tsconfig.json` that
`references` all of the packages, including any apps. To do so, all
dependent `tsconfig.json`s must set `"composite": true`.

When `"composite": true` is set in a package, Typescript does
portability analysis on all exports to ensure that Typescript will be
able to properly determine the type of those exports if they are
imported into a consuming package as a dependency. This is true even if
that package never ultimately exports that type through its `main` or
`exports` fields, or is private, or is not depended on by any other
projects. This portability check happens _even if checkJs is disabled_.

Because `importMap` relies on type inference on transient dependencies,
Typescript requires a type annotation to conclude that its type is
portable.

Ultimately, there's no reason for Payload consumers to care about or
enforce the type of `importMap`. It's automatically generated by
Payload, never hand edited, and only consumed by other auto-generated
Payload code.

Since Payload already has a type for `ImportMap`, we can type
`importMap` with that type. It will satisfy the type, and is also
considered portable.

### How?

Updated the generateImportMap template.

Fixes #9914

Co-authored-by: Paul Popus <paul@payloadcms.com>

Author: smoores-dev

packages/payload/src/bin/generateImportMap/index.ts

@@ -155,6 +155,7 @@ export async function writeImportMap({
 
   const importMapOutputFile = `${imports.join('\n')}
 
+/** @type import('payload').ImportMap */
 export const importMap = {
 ${mapKeys.join(',\n')}
 }