[dotnet-watch] Clean up subcommands parsing

Once dotnet CLI refactoring is done we can:
- remove dependency on dotnet project
- create WatchCommandDefinition
- parameterize created test commands to allow better testing
E.g. Currently we don't test MTP since to just parse the test command we need to set up environment global.json above the current directory (current test dir is `artifacts\bin\dotnet-watch.Tests\Debug\net10.0`). We should be able to select MTP command instead of the generic test command as needed. Update test: `ForwardedBuildOptions_Test` to check both MTP and VSTest.