chore: update .gitignore

[jpnurmi]

VS Code C#/DevKit/Uno extensions notice the .NET project in tests/fixtures/dotnet_signal and like to leave behind annoying .csproj.user files.

$ git status
On branch master
Your branch is up to date with 'upstream/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        tests/fixtures/dotnet_signal/test_dotnet.csproj.user

nothing added to commit but untracked files present (use "git add" to track)