Test coverage metrics (Codecov)

tried codecov before, didn't work (can't remember why), but they've recently rewritten their tool https://about.codecov.io/blog/new-cli-from-codecov-new-features-and-coverage-in-your-terminal/

for TASD.NET, I tried the second-party tool coverlet, and that works locally, just need to check GitLab CI (it has 5 options for <CoverletOutputFormat/> but I think cobertura is correct)

• update: seems to be broken now (because we switched to .NET Framework for tests?) https://github.com/coverlet-coverage/coverlet/issues/1404#issuecomment-2453160272
• coverlet says it can merge outputs but I tried and it's borked, gonna have to invoke dotnet tool after build... maybe wait for .slnx since I think it's easier there
• also used by AssetRipper (.NET 9)

for TASVideos.org I tried the first-party tool and got it working, see https://github.com/TASVideos/tasvideos/commit/d181fa3ed96068f49bcacb1508702c487a5db8a6 and prior commits