Add header-units.json

[Stephan T. Lavavej (StephanTLavavej)]

Works towards #60.

This adds header-units.json, a file that will tell the compiler which headers are eligible to be automatically treated as header units at the user's request. (That is, there will be a compiler option allowing #include <vector> to be treated as import <vector>; without changing the source code.)

Changing .vscode/settings.json is necessary because otherwise the "**/stl/inc/**" entry causes this file to be displayed as C++.

Remaining tasks:

• Name the file header-units.json with a hyphen.
• Confirm that JSON With Comments is acceptable.
• Confirm that this syntax is correct.
• Update stl/CMakeLists.txt to copy this to the output directory.
  • Verified for x86 and x64 that this copies to out\build\ARCH\out\inc\header-units.json.
• Update internal MSBuild machinery to copy this to the output directory.
  • Updated src/vctools/crt/copy_crt/copy_crt.nativeproj.
• Update internal setup authoring.
  • Updated src/SetupPackages/swix/VisualCpp/crt.headers/files.swr.
• Confirm that internal src\vctools\crt\lkgsync\lkgsync.xml should ignore this.
  • Yep; Amy Wishnousky (@amyw-msft) will add this to the LKG12 todo list, as this will be low priority for a while.
• Update the wiki's Files To Edit When Adding Or Removing Files.

[Cameron DaCamara (cdacamar)]

As per the design doc agreed upon between the FE, project system, and Libs, the syntax is indeed correct.