build(presets): Major CMakePresets overhaul, unify vcpkg, modernize VC6 support

- Deduplicate configure presets for easier maintenance
- Make vcpkg the default and speed up dependency installation for all builds
- Enable vcpkg integration for VC6 builds
- Switch VC6 builds to Ninja Multi-Config: optimization level can now be switched without reconfiguring
- Enable line info for fast VC6 release builds
- Re-enable debug info for VC6 configurations
- Add RelWithDebInfo configs to all presets, including VC6
- VC6 environment now requires only the VS6_DIR environment variable to be set
- Decouple dev builds from optimization config for more flexible development

Author: aliendroid1