chore: restructure repo

[alxdca]

Summary

• make the repository root the single @arbitrum/chain-sdk package
• remove the redundant package boundary under src
• build and publish from the root dist directory
• update workspace links and lockfile importer metadata
• preserve CommonJS output while marking ESM-based ESLint and Vitest configurations explicitly

Why / Context

The repository previously had a private root package alongside the publishable package under src. This added an unnecessary workspace and package boundary, split package metadata across two manifests, and required build output under src/dist.

This change consolidates package metadata, dependencies, scripts, exports, and publishing configuration at the repository root. Unrelated TypeScript resolver, declaration, and dependency migrations were intentionally left out.