Allow configuring npmClient on a project level

Currently, the npmClient ionic uses can only be set globally, using e.g.:
```sh
ionic config set -g npmClient pnpm
```

In my opinion, it should be possible to override this on a project level, e.g.:
```sh
ionic config set npmClient pnpm
```
This commands "succeeds" and updates the `ionic.config.json`, but ionic doesn't check that file for this parameter (a bit confusing).

The use cases:
- We have some older projects that use npm, and newer once use pnpm. It has happened multiple times that I run `ionic build` when i just switched project and forgot to switch package manager. This then creates a `pnpm-lock` file in an `npm` project. And regularly breaks my dependencies and requires deleting `node-modules` and reinstalling. This wastes time.
- We have pipelines that build iOS projects on local MacMini's. However, if the global ionic package manager changes, and a pipeline doesn't have the setting to change it back, the build breaks. It's not always obvious that the issue is caused by a switch in package manager.

This was previously suggested in https://github.com/ionic-team/ionic-cli/issues/4570, but was not implemented because the lack of use cases.

先追蹤現有的 `ionic config set` 對全域設定和專案層級 `ionic.config.json` 的處理方式，然後了解 `ionic build` 如何選擇其套件管理器。確認專案設定與全域設定之間所需的優先順序，並驗證建置會使用專案設定，而不會建立錯誤的 lockfile。

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