Remove Yarn

[ovidiuch]

Why?

• Yarn 1.x is deprecated and has bugs that won't be resolved.
• Using NPM is one less dependency because it's the standard package manager.
• Windows GitHub Actions are sped up significantly after moving away from Yarn 1.x.

TODO

• Codecov token.
• Use npm ci as yarn --frozen-lockfile equivalent.
• Cache NPM cache (node_modules is deleted before calling npm ci).
• Cache Playwright binaries.
• Review test-pnp workflow.
• Remove BerniWittmann/background-server-action.
• Mention "optionalDependencies" NPM workaround in DEBT doc.
• Update release-canary workflow.
• Upgrade to Lerna 8.

Related:

• [BUG] Platform-specific optional dependencies not being included in package-lock.json when reinstalling with node_modules present npm/cli#4828

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