feat: add yarnpkg/nm/hoist export (#6611)

## What's the problem this PR addresses?

Bundling `@yarnpkg/nm` (due to subdeps) is large and not needed when one
needs only a few exports from `./hoist` file:
https://github.com/yarnpkg/berry/blob/b3dfa91abd591a3d572daadfdba54c671c7d882b/packages/yarnpkg-nm/sources/buildNodeModulesTree.ts#L1-L4

But `hoist.ts` is a zero-dep lib:

https://github.com/yarnpkg/berry/blob/b3dfa91abd591a3d572daadfdba54c671c7d882b/packages/yarnpkg-nm/sources/hoist.ts#L1-L3

It's already built/distrubuted separately (see
https://www.npmjs.com/package/@yarnpkg/nm?activeTab=code), just an entry
in package.json was missing

Both its exports (and types) are already re-exported:

https://github.com/yarnpkg/berry/blob/b3dfa91abd591a3d572daadfdba54c671c7d882b/packages/yarnpkg-nm/sources/index.ts#L23-L24

So this is not increasing public API surface, just adds a more
convenient way to import it without pulling in the whole of
`@yarnpkg/core` and `@yarnpkg/pnp`

## How did you fix it?

Added `./hoist` to exports to point to `./lib/hoist.js`

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.yarn/versions/ce211f43.yml

@@ -0,0 +1,6 @@
+releases:
+  "@yarnpkg/nm": patch
+
+declined:
+  - "@yarnpkg/plugin-nm"
+  - "@yarnpkg/pnpify"

packages/yarnpkg-nm/package.json

@@ -5,6 +5,7 @@
   "main": "./sources/index.ts",
   "exports": {
     ".": "./sources/index.ts",
+    "./hoist": "./sources/hoist.ts",
     "./package.json": "./package.json"
   },
   "sideEffects": false,
@@ -23,6 +24,7 @@
     "main": "./lib/index.js",
     "exports": {
       ".": "./lib/index.js",
+      "./hoist": "./lib/hoist.js",
       "./package.json": "./package.json"
     }
   },