VS: Convert flag tables to JSON files instead of hard-coded data
Move flag tables out from embedded C++ headers and into JSON files as initial move towards the support mentioned in !163 (merged). As part of this, move cmIDEFlagTable to use std::strings rather than raw pointers and change the termination condition from nullptr entry to empty entry.
Now including base versions with the json script and commits for the modifications from there. I expect that we may want to squash those later, but it might be easier to review like this.
Topic-rename: vs-json-flag-table