chore: migrate from yarn to pnpm

[filipsobol]

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This repository currently uses Yarn 1 (classic), which is no longer supported. I suggest migrating to PNPM, which:

• offers better performance,
• saves disc space,
• allows updating dependencies, even outside the range specified in package.json,
• allows managing and switching between Node versions,
• can automatically resolve git conflicts in its lock file,
• prevents using dependencies not registered in the package.json.

Same as Yarn, it:

• supports workspaces,
• supports .npmrc file

Dependabot doesn’t support pnpm yet, but we can use renovatebot for the time being.