t8code does not compile with -Wextra and/or -Wpedantic

[jmark]

Would be nice to fully to support these flags, too.

[lukasdreyer]

Those are the additional warnings that arise:

-Werror=unused-parameter, fixed by introducing [[maybe_unused]] https://en.cppreference.com/w/cpp/language/attributes/maybe_unused

-Werror=type-limits, fixed by removing the unnecessary unsigned int >= 0 assertion

-Werror=implicit-fallthrough=, fixed by not allowing switch statements to fall through

-Werror=ignored-qualifiers, fixed by removing const return qualifier

[holke]

#1158 fixes parts of the fallthroughs.

[holke]

#1118 is related