Migrate to PNPM

[joe-bell]

Description

• Migrate from Lerna + Yarn to pnpm
• Migrate existing scripts
• Update 11ty example (outdated Node version on Vercel)
• Update CONTRIBUTING

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Follow the Style Guide.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).

[vercel]

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Name	Status	Preview	Updated
plaiceholder-docs	✅ Ready (Inspect)	Visit Preview	Jun 12, 2022 at 11:30PM (UTC)
plaiceholder-with-11ty	✅ Ready (Inspect)	Visit Preview	Jun 12, 2022 at 11:30PM (UTC)
plaiceholder-with-next	✅ Ready (Inspect)	Visit Preview	Jun 12, 2022 at 11:30PM (UTC)