DRY GHA workflows

[cte]

Description

I've been tinkering with GHA for evals, and this was bugging me.

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Important

Introduces a reusable GitHub Action for setting up Node.js and pnpm, streamlining multiple workflow files.

• New Action:
  • Adds .github/actions/setup-node-pnpm/action.yml to setup Node.js and pnpm with caching and dependency installation.
  • Supports inputs for node-version, pnpm-version, skip-install, and install-args.
• Workflow Changes:
  • Replaces individual Node.js and pnpm setup steps with ./.github/actions/setup-node-pnpm in changeset-release.yml, code-qa.yml, marketplace-publish.yml, nightly-publish.yml, and update-contributors.yml.
  • Removes NODE_VERSION and PNPM_VERSION environment variables from workflows.
• Misc:
  • Removes redundant pnpm installation steps in workflows.

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