MAINT migrate from `npm` to `pnpm`

[Charlie-XIAO]

Because my disk is going to explode... Also there will be nicer diff for the lock file since it's yaml instead of json.

And I heard that pnpm has nice features for monorepos which is our case (core and several tooling packages), though I haven't explored much yet, except that now pnpm install at the project root will recursively install all dependencies for the workspaces due to the pnpm-workspace.yaml configuration which is quite nice already.

There are many things to look into for our monorepo layout, e.g. I'm planning to refactor the scripts in package.json and maybe use some tool like turborepo to ensure same versions of dependencies, make better pipelining both for building and publishing, etc. But those are future work.

[!NOTE] Make sure you delete all your node_modules (core, website, and all tooling packages) before running pnpm install.

[!NOTE] The commands that used to be npm run xxx will all become pnpm xxx now. For instance, pnpm tauri dev and pnpm tauri build -- --debug.

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