Ninja-Multiconfig: The default build preset should support Ninja-Multiconfig

### Brief Issue Summary

As discussed: https://github.com/microsoft/vscode-cmake-tools/discussions/2764#discussioncomment-3927358

The default build preset option could provide extended build presets with Debug, Release in case the generator is a Ninja-Multiconfig or at least issue a warning.

Start by reading the linked discussion for the expected Ninja-Multiconfig behavior. Then locate the default build preset handling and examine how generator-specific configurations are represented. Done means the default option supports Debug and Release for Ninja-Multiconfig or emits the proposed warning, with behavior covered by the relevant tests.

Written by the indexing model from the issue text.