Migrate to modern .NET with SDK-style projects for cross-platform support

[Copilot]

The solution targeted .NET Framework 4.5 using legacy project formats, preventing builds on systems without Windows-specific framework packs and limiting cross-platform deployment.

Changes

Project Structure

• Core: Converted to SDK-style project with multi-targeting (.NET Standard 2.0 + .NET 8.0)
• Terminal: Converted to SDK-style project targeting .NET 8.0
• Removed obsolete packages.config and App.config files

Dependencies

• Added System.IO.Ports 8.0.0 for cross-platform SerialPort support
• Updated CommandLineParser 1.9.71 → 2.9.1

Code Updates

• Updated Terminal to use CommandLineParser 2.x API (removed DefaultValue attribute, HelpOption method)
• Fixed broken Terminal example code (incorrect API usage with ModbusRtuSession constructor and SendRequest overload)

Before/After

Before:

// Didn't compile - missing slaveAddress in constructor
var session = new ModbusRtuSession(new ModbusProtocolImpl(stream));
var response = session.SendRequest<ResponseFunc03>(1, 3, func03); // Wrong signature

After:

var session = new ModbusRtuSession(new ModbusProtocolImpl(stream), slaveAddress: 1);
var response = session.SendRequest<ResponseFunc03>(functionCode: 3, func03);

The library now builds on Windows, Linux, and macOS with modern .NET tooling.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Modern .NET support</issue_title> <issue_description>Hi! Your solution lacks modern .NET support, which limits cross-platform compatibility. Please consider adding support for recent .NET versions to improve usability across different platforms.

Thank you!</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes sontx/modbus#4

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