Add GitHub Actions CI workflow to replace AppVeyor

[Copilot]

Overview

This PR replaces the existing AppVeyor CI configuration with a modern GitHub Actions workflow, providing comprehensive multi-platform testing and automated package building.

What Changed

1. New GitHub Actions Workflow

Created .github/workflows/dotnet-ci.yml with the following features:

• Multi-platform testing: Runs builds and tests on Ubuntu, Windows, and macOS
• Multi-.NET version support: Tests with .NET 6.0, 8.0, and 9.0 SDKs
• Comprehensive framework coverage: Builds all target frameworks (netstandard2.0, netstandard2.1, net472, net6.0, net8.0, net9.0)
• Automated NuGet packaging: Creates packages on main branch pushes
• Test artifact uploads: Preserves test results for all platforms
• Performance optimizations: Implements NuGet package caching

The workflow consists of two jobs:

1. build-and-test: Validates the build across all three operating systems using a matrix strategy
2. package: Creates NuGet packages (runs only on main branch pushes after successful tests)

2. Removed AppVeyor Configuration

• Deleted appveyor.yml
• Removed reference from solution file

3. Updated Documentation

• Updated build status badges in README.md and README_EN.md to point to the new GitHub Actions workflow

4. Fixed Build Issue

Fixed a syntax error in Chsword.Excel2Object/Internal/ExcelConstants.cs where the CommonDateTimeFormats array was missing its opening bracket for the collection expression.

Why This Change?

The new GitHub Actions CI provides several advantages over AppVeyor:

1. Better platform coverage: Tests on Linux, Windows, and macOS instead of just Windows
2. Native integration: Built into GitHub with better visibility and no external dependencies
3. Free for open source: No concerns about CI minutes or service availability
4. Modern tooling: Latest action versions and better caching mechanisms
5. Faster feedback: Parallel execution across platforms

Testing

✅ All 88 tests pass on the local environment
✅ Builds successfully for all 6 target frameworks
✅ Workflow YAML syntax validated with yamllint
✅ Clean build from scratch verified

The workflow will be automatically triggered on the next push/PR to validate multi-platform compatibility.

Related Issues

Fixes issue: 添加 github ci 方案 取代现有ci 并保证项目编译测试通过 (Add GitHub CI solution to replace existing CI and ensure project builds and tests pass)

Original prompt

添加 github ci 方案 取代现有ci 并保证项目编译测试通过

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[Copilot]

Pull Request Overview

This PR modernizes the CI/CD pipeline by replacing the existing AppVeyor configuration with a comprehensive GitHub Actions workflow that provides multi-platform testing and automated NuGet package building.

• Replaces AppVeyor CI with GitHub Actions for better platform coverage and native GitHub integration
• Implements multi-platform testing across Ubuntu, Windows, and macOS with .NET 6.0, 8.0, and 9.0 support
• Fixes a syntax error in the ExcelConstants.cs file for modern C# collection expressions

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
appveyor.yml	Removes legacy AppVeyor CI configuration
.github/workflows/dotnet-core.yml	Removes outdated .NET Core workflow
.github/workflows/dotnet-ci.yml	Adds comprehensive CI workflow with multi-platform testing and NuGet packaging
README.md, README_EN.md	Updates build status badges to point to new GitHub Actions workflow
Chsword.Excel2Object.sln	Removes AppVeyor configuration reference from solution
Chsword.Excel2Object/Internal/ExcelConstants.cs	Fixes collection expression syntax for C# 12 compatibility

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[Copilot]

The condition only checks for 'main' branch but the workflow triggers on both 'main' and 'master' branches (line 5). Consider updating to check for both branches: (github.ref == 'refs/heads/main' || github.ref == 'refs/heads/master')

Suggested change

	if: github.event_name == 'push' && github.ref == 'refs/heads/main'
	if: github.event_name == 'push' && (github.ref == 'refs/heads/main' || github.ref == 'refs/heads/master')