Add pnpm support

[steffendietz]

Hi, thanks for creating foxy.

I recently added pnpm support to foxy, which I am currently using successfully in a personal project.

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Updating dependencies                                 
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Installing dependencies from lock file (including require-dev)
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Generating autoload files
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Merging Composer dependencies in the asset package
Installing pnpm dependencies
Progress: resolved 1, reused 0, downloaded 0, added 0
Packages: +3
+++
Packages are hard linked from the content-addressable store to the virtual store.
  Content-addressable store is at: /workspaces/laminas-turbo-test/.pnpm-store/v3
  Virtual store is at:             node_modules/.pnpm
Progress: resolved 3, reused 0, downloaded 2, added 2

dependencies:
+ @hotwired/turbo 7.3.0
+ bootstrap 4.6.2 (5.2.3 is available)
+ jquery 3.6.4

 WARN  Issues with peer dependencies found
.
└─┬ bootstrap 4.6.2
  └── ✕ missing peer popper.js@^1.16.1
Peer dependencies that should be installed:
  popper.js@^1.16.1

Done in 1.7s
Progress: resolved 3, reused 0, downloaded 3, added 3, done
No security vulnerability advisories found

I thought I would share it. This should also close #46.

[francoispluchino]

@steffendietz Thank you for your PR!