refactor [NET-1606]: Make `utils` package work without polyfilling

[mondoreale]

This pull request introduces significant improvements to the @streamr/utils package, focusing on robust browser and Node.js environment support, build configuration enhancements, and codebase modernization. The main changes include a new dual build and export system for browser and Node.js, environment variable handling refactoring, and the replacement of Node.js-specific APIs with cross-platform polyfills.

Changes

Build and packaging enhancements:

• Added a dual build system using a new rollup.config.mts to generate separate browser and Node.js bundles, with appropriate module resolution and aliasing for each environment. (packages/utils/rollup.config.mts)
• Updated package.json to use the exports field for conditional exports (browser vs. Node.js), adjusted the files array, and added new build scripts and dependencies for Rollup and polyfills. (packages/utils/package.json) [1] [2]
• Introduced a prebuild and postbuild step to streamline the build process and ensure clean output. (packages/utils/package.json)

Cross-platform and polyfill improvements:

• Replaced direct Node.js API usage (e.g., crypto, os) with cross-platform abstractions and browser polyfills (e.g., buffer-shim, path-browserify, custom os and env shims for browsers). (packages/utils/src/browser/crypto.ts, packages/utils/src/browser/env.ts, packages/utils/src/browser/os.ts, packages/utils/src/exports-browser.ts, packages/utils/src/keyToArrayIndex.ts, packages/utils/karma.config.ts) [1] [2] [3] [4] [5] F8929faeL1)
• Removed the old crossPlatformCrypto.ts in favor of new environment-specific implementations. (packages/utils/src/crossPlatformCrypto.ts)

Environment variable handling:

• Refactored all environment variable access to use a unified env abstraction, ensuring safe access in both Node.js and browser contexts. (packages/utils/src/Logger.ts, packages/utils/src/browser/env.ts) [1] [2] [3] [4] [5] [6]

Testing and configuration:

• Updated Jest and Karma configurations to support new module aliases and environment-specific code paths. (packages/utils/jest.config.ts, packages/utils/karma.config.ts) (packages/utils/jest.config.tsL1-R11, F8929faeL1)

Code modernization and cleanup:

• Updated cryptographic utilities to use new cross-platform imports and types, improving maintainability and browser compatibility. (packages/utils/src/SigningUtil.ts, packages/utils/src/exports.ts) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Additional minor improvements:

• Added @streamr/utils to the Docker build bootstrap step. (Dockerfile.node)

These changes collectively make the @streamr/utils package more robust, easier to consume in different environments, and simpler to maintain.

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[!NOTE] Modernizes @streamr/utils to be environment-agnostic with separate Node and browser builds and conditional exports.

• Introduces dual build pipeline via rollup.config.mts and package.json exports for browser/Node outputs
• Replaces Node-only APIs with cross-platform abstractions: new @/crypto, @/env, browser shims (buffer-shim, path-browserify, custom os), removes crossPlatformCrypto
• Refactors Logger, SigningUtil, keyToArrayIndex, and exports to use new env/crypto utilities
• Updates Jest/Karma and TS configs for path aliases and environment-specific sources; adds utils to Docker bootstrap

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