fix(build): enable tokio process feature for the async feature

The async feature pulled in tokio with no features enabled, so
tokio::process was unavailable and any build with --features async failed
to compile (run_async could not construct a tokio::process::Command).
Route tokio/process through the async feature so it is enabled exactly
when async is.

Author: kaniini

Cargo.lock

@@ -23,7 +23,7 @@ tokio = { version = "1.52.3", optional = true }
 tempfile = "3"
 
 [features]
-async = ["dep:tokio"]
+async = ["dep:tokio", "tokio/process"]
 
 [lib]
 name = "styrolite"