deps: migrate d2.js to d2lang npm scope

[alixander]

Summary

• switch the playground dependency and imports from @terrastruct/d2 to canonical @d2lang/d2
• update the frozen Yarn 1 lock entry to the verified public 0.1.33 tarball
• point the README package link at the new namespace

Validation

• Yarn 1.22.22 frozen-lockfile install
• production JS/CSS builds with current and project-pinned esbuild
• real D2 compile/render smoke test
• go test ./...
• go build ./...
• go vet --composites=false ./...
• Prettier and git diff --check