It's annoying for downstream users when they don't use the exact same
compiler as we do because any random new (or old) compiler warning
breaks the build.

Only set the flag on CI or when explicitly requested with the new
QJS_BUILD_WERROR cmake option.

Fixes: #1339

The return type of strrchr depends on the type of the first argument in
C23. Not that quickjs-ng is a C23 project at this time but it's easy to
fix.

Fixes: #1333

This is noticeable when compiling large regular expressions when there
is a memory limit in effect: lre_compile succeeds but subsequently
trying to copy its output to a new string fails.

The new indirect string kind will likely be useful in more places.