Set vaadin.pnpm.enable = true

[msabramo]

Without this, when I start the app, npm runs and pnpm-lock.yaml is
ignored and actually gets deleted and a package-lock.json is generated:

$ git status
On branch main
Your branch is up to date with 'origin/main'.

Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   .factorypath
        deleted:    pnpm-lock.yaml

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        package-lock.json

I think this results in getting an incorrect set of package versions,
because I get a TypeScript compile error about IReactionOptions in
frontend/view.ts:

ERROR in /Users/abramowi/learn/mobx/vaadin-fusion-mobx/frontend/view.ts
./view.ts
[tsl] ERROR in /Users/abramowi/learn/mobx/vaadin-fusion-mobx/frontend/view.ts(22,12)
      TS2314: Generic type 'IReactionOptions' requires 2 type argument(s).
Child InjectManifest:
     1 asset
    Entrypoint InjectManifest = sw.js
       78 modules
ℹ ｢wdm｣: Failed to compile.

When I set vaadin.pnpm.enable, then it seems to use pnpm instead of
npm and then I don't end up getting the compile error.