Ninja Build Backend for DUB

This project adds a Ninja build backend to DUB, the D package manager and build tool used by the D ecosystem. Currently, DUB primarily drives builds via direct compiler invocations. This limits incremental build performance, parallelism, and integration with modern build tooling. The proposal introduces a backend that allows DUB to generate Ninja build files and delegate compilation to Ninja. This enables faster incremental builds, better dependency tracking, improved parallel execution, and easier IDE/toolchain integration.