feat(core): ldml C++ feature support 🙀

- make ldml.cpp print FAILED on failure especially because of https://github.com/microsoft/vscode-cpptools/issues/487
- add vkey validation (#7135)
- add disp validation
- fix: a latent bug in disp! disp.count was === vkey.count, oops. Fix and test cases.

For: #7985