Multi-config Ninja support

This is an umbrella issue for recording the remaining tasks for multi-config Ninja support to be considered complete (or at least comparable to single-config Ninja support). It has been split out from the description of !4086 (merged) which provided the initial multi-config Ninja support so that it can be tracked as an open issue.

TODO:

• Add a nightly build with this generator
• Unity build support
• Precompile header support (!4201 (merged))
• Swift support (!4440 (merged))
• CUDA support (!4187 (merged))
• Multi-config packaging with cpack -C
• Qt support (e.g. the <ORIGIN>_autogen output directory and associated include path handling)
• Disable cross-configuration build when not desired (building foo:Release from build-Debug.ninja) (!4205 (merged))
• Add variable to control default config alias in build.ninja (!4238 (merged))
• Add variable to granularly control configs generated in cross-config mode (!4269 (merged))
• Fix support for Ninja 1.10: #20247 (closed)