refactor(clientonly): modernize code structure and add comprehensive tests

[KristjanESPERANTO]

This PR improves clientonly start option with better code structure, validation, and comprehensive test coverage.

Changes

Refactoring:

• Improved parameter handling with explicit function parameter passing instead of closure
• Added port validation (1-65535) with proper NaN handling
• Removed unnecessary IIFE wrapper (Node.js modules are already scoped)
• Extracted getCommandLineParameter as a reusable top-level function
• Enhanced error reporting with better error messages
• Added connection logging for debugging

Testing:

• Added comprehensive e2e tests for parameter validation
• Test coverage for missing/incomplete parameters
• Test coverage for local address rejection (localhost, 127.0.0.1, ::1, ::ffff:127.0.0.1)
• Test coverage for port validation (invalid ranges, non-numeric values)
• Test coverage for TLS flag parsing
• Integration test with running server

Testing

All tests pass:

npm test -- tests/e2e/clientonly_spec.js
# ✓ 18 tests passed

[khassel]

Not directly related to this pull request, but I've been wondering whether we need these subdirectories for clientonly and serveronly. We could put everything under js and simply change the call in package.json for serveronly. And we could do the same for clientonly.

But maybe I've overlooked something that argues against it...