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

[Copilot]

Summary

Refactors @streamr/utils to eliminate polyfill requirements by implementing platform-specific builds for Node.js and browser environments using Rollup, with separate crypto/MD5 implementations and module aliasing.

Changes

Build system and packaging:

• Dual-build system using Rollup to generate separate Node.js (ESM/CJS) and browser bundles
• Package exports configured for conditional loading based on environment
• Simplified build pipeline: removed tsconfig.rollup.json, use tsx to execute .mts config directly
• Fixed rollup config to reference compiled output in dist/ rather than source paths
• TypeScript project references for Node, browser, Jest, and Karma builds

Cross-platform crypto and environment:

• Platform-specific crypto implementations (src/node/crypto.ts, src/browser/crypto.ts)
• Platform-specific MD5 hashing (src/node/md5.ts, src/browser/md5.ts)
• Environment variable abstraction (src/node/env.ts, src/browser/env.ts) replaces direct process.env access
• Browser shims for os and buffer modules
• Safe crypto global check using globalThis to handle non-secure contexts

Code quality improvements:

• Keccak instance now local const instead of module-level mutable variable
• Jest config aligned with tsconfig to use Node.js implementations in tests
• Module aliases (@crypto, @md5, @/) configured across Jest, Karma, and Rollup

Limitations and future improvements

• Rollup config must be pre-compiled to .mjs for Node 20 compatibility (.mts direct execution fails)
• Browser crypto requires secure context (HTTPS); graceful degradation could be improved

Checklist before requesting a review

• Is this a breaking change? If it is, be clear in summary.
• Read through code myself one more time.
• Make sure any and all TODO comments left behind are meant to be left in.
• Has reasonable passing test coverage?
• Updated changelog if applicable.
• Updated documentation if applicable.

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[linear]

NET-1606 `@streamr/utils` used in a Vite/webpack app requires polyfilling

[mondoreale]

I'll copy over the new desc. Cheers.