Modernize to .NET 8: add SDK-style projects, CI and NuGet publish workflow

[forcewake]

Motivation

• Move the repository to a modern-only strategy targeting .NET 8 and prepare a v2 breaking release.
• Replace legacy build matrix and msbuild/psake-based pipeline with SDK-style projects and dotnet-based tooling.
• Improve runtime performance and reduce allocations in core reflection/conversion/parser paths.

Description

• Added new SDK-style projects under src/*/*.Modern for FlatFile.Core, Core.Attributes, Delimited, FixedLength and their attribute variants targeting net8.0 with version 2.0.0 and a Directory.Build.props to centralize package metadata and README packaging.
• Implemented runtime improvements in core code: replaced reflection DynamicInvoke cache with compiled expression factories and ConcurrentDictionary caches in ReflectionHelper, added converter caching and invariant-culture conversion in TypeChangeExtensions, and small parser/trim optimizations in DelimitedLineParser and FixedLengthLineParser.
• Updated build automation: converted assets/psake-common.ps1 tasks to use dotnet restore/clean/build for modern projects, added docs/modernization-plan.md, and updated README.md to document modernization and NuGet publishing.
• Added GitHub Actions workflows: .github/workflows/ci.yml to run dotnet build for modern projects on push/PR and .github/workflows/publish-nuget.yml to dotnet pack and push packages to NuGet on master (uses NUGET_API_KEY secret and generates package version 2.0.<run_number>).

Testing

• No existing unit tests were modified as part of this change and no automated unit-test runs were executed in this PR.
• A CI workflow (CI) was added that will run dotnet build for the modern projects on pushes and pull requests, but this workflow was added (not executed) by the change itself.
• A publish workflow (Publish NuGet (v2)) was added to pack and push .Modern projects; it validates that NUGET_API_KEY is configured before publishing.
• Local or CI build validation should be performed after adding the repository NUGET_API_KEY secret and/or on subsequent pushes to verify packaging and publishing steps.

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Codex Task