new: dev: add <isc/bit.h>

The `<isc/bit.h>` header is a GNU C11 compatible version of C23's
`<stdbit.h>`.

It currently uses either `<stdbit.h>` or the equivilent compiler
builtins. However, the generic `__builtin_ctzg` and `__builtin_ctlz`
builtins are not available in every compiler version and thus falls
back to manually selecting from type.

Furthermore, the ctz fallback has been removed since `__builtin_ctzll`
has been used for a while directly without any compilation issues from
users. Thus, we can also require `__builtin_ctz`.

Merge branch 'aydin/isc_bit' into 'main'

See merge request isc-projects/bind9!10282