chore: move pnpm settings to pnpm-workspace.yaml (#791)

## Summary

- Move `pnpm.overrides` and `pnpm.peerDependencyRules` out of
`package.json` and into `pnpm-workspace.yaml` as top-level `overrides`
and `peerDependencyRules` keys.
- pnpm no longer reads the `pnpm` field in `package.json` and emits a
warning on install:

  ```
[WARN] The "pnpm" field in package.json is no longer read by pnpm. The
following keys were ignored: "pnpm.overrides",
"pnpm.peerDependencyRules". See https://pnpm.io/settings for the new
home of each setting.
  ```

## Test plan

- [x] `vp install` no longer emits the deprecation warning
- [x] `vitest` still resolves to `@voidzero-dev/vite-plus-test` via the
override

Author: fengmk2

package.json

@@ -111,17 +111,5 @@
   "engines": {
     "node": ">= 22.0.0"
   },
-  "packageManager": "pnpm@11.3.0",
-  "pnpm": {
-    "overrides": {
-      "vite": "npm:@voidzero-dev/vite-plus-core@^0.1.22",
-      "vitest": "npm:@voidzero-dev/vite-plus-test@^0.1.22"
-    },
-    "peerDependencyRules": {
-      "allowAny": [
-        "vite",
-        "vitest"
-      ]
-    }
-  }
+  "packageManager": "pnpm@11.3.0"
 }

pnpm-lock.yaml

@@ -10,3 +10,10 @@ minimumReleaseAgeExclude:
   - '@voidzero-dev/vite-plus-win32-arm64-msvc'
   - '@voidzero-dev/vite-plus-win32-x64-msvc'
   - vite-plus
+overrides:
+  vite: 'npm:@voidzero-dev/vite-plus-core@^0.1.22'
+  vitest: 'npm:@voidzero-dev/vite-plus-test@^0.1.22'
+peerDependencyRules:
+  allowAny:
+    - vite
+    - vitest