rust: macros: support `proc-macro2`, `quote` and `syn`

One of the two main uses cases for adding `proc-macro2`, `quote` and
`syn` is the `macros` crates (and the other `pin-init`).

Thus add the support for the crates in `macros` already.

Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-21-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>