D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082
Listing all languages inside meson.build for the Yelp-based manual
localisation is error-prone, and it also requires parsing and modifying
Meson files from external tools.
Just like we do for i18n, we can use an ancillary `LINGUAS` file in the
help source sub-directory to list all the help languages.