debug visualizer for gcc (and/or clang)

[vinniefalco]

Similar to json.natvis

[grisumbras]

https://sourceware.org/gdb/current/onlinedocs/gdb/Pretty-Printing.html#Pretty-Printing

[grisumbras]

Examples:
https://undo.io/resources/gdb-watchpoint/debugging-pretty-printers-gdb-part2/
nlohmann/json#1952

[grisumbras]

Implemented in #733.