Update framework and Modernize setup

[Avasam]

Modernize project, build, and tooling

Project / build

• Convert to SDK-style .csproj; retarget .NET Framework 4.6.1 → 4.8.1 (matches the LiveSplit host).
• Enable UseWindowsForms; drop legacy ClickOnce/bootstrapper cruft and explicit Compile/EmbeddedResource items (SDK globbing).
• Vendor the LiveSplit reference assemblies (LiveSplit.exe, LiveSplit.Core.dll, UpdateManager.dll) under lib/ with Private=false, replacing hardcoded absolute HintPaths. Added lib/README.md with refresh instructions (pinned to LiveSplit 1.8.37).
• Generate assembly metadata from csproj properties; remove Properties/AssemblyInfo.cs.
• Add packages.lock.json for reproducible restores.

Solution / versioning

• Replace .sln with .slnx, then drop it entirely (single-project repo builds fine from the .csproj under both VS 2022 and the dotnet CLI).
• Single source of truth for the version: csproj <Version>; AssemblyVersion/FileVersion derive from it and the factory reads it at runtime.

Auto-updater

• Repoint updater URLs from the archived KaDiWa4 repo to Toufool/LiveSplit.AutoSplitIntegration @ main.
• Bump to 1.8.3 and add a matching update.xml entry.

CI / tooling

• Add GitHub Actions build workflow (windows-2022, .NET SDK 10, build + dotnet format check, upload DLL artifact).
• Apply shared configs (.editorconfig, .vscode/), remove custom .gitattributes overrides, and trim .gitignore from ~248 lines of boilerplate to project-relevant entries.

Docs

• Rewrite README "Compiling" (SDK 10 + 4.8.1 dev pack, dotnet build or VS 2022) and add a "Releasing a new version" section.

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LLM/Coding agent used back-and-forth to massively speed up the process and analyze through the cruft. Leaving as draft until I properly finish reviewing all the text changes and "humanize" PR description in my own words. Code and config changes already fully human reviewed.