Add split-debuginfo profile option

Add split-debuginfo profile option

This commit adds a new `split-debuginfo` option to Cargo compilation
profiles which gets forwarded to the `-Csplit-debuginfo` codegen option
in rustc. This commit also sets the default, only on macOS, to be
`-Csplit-debuginfo=unpacked`. The purpose of this change is to leverage
rust-lang/rust#79570 to avoid running `dsymutil` on incremental builds
while also preserving a pleasant debugging experience by default. This
should lead to much faster incremental build times on macOS since
`dsymutil` isn't exactly the speediest tool in the world.

This is technically a breaking change in Cargo because we're no longer
by-default producing the `*.dSYM` folders on macOS. If those are still
desired, however, authors can always run `dsymutil` themselves or
otherwise configure `split-debuginfo = 'packed'` in their
manifest/profile configuration.