Make which-support feature non-optional (#13125)

# Description
Removes the `which-support` cargo feature and makes all of its
feature-gated code enabled by default in all builds. I'm not sure why
this one command is gated behind a feature. It seems to be a relic of
older code where we had features for what seems like every command.

tests/shell/mod.rs

@@ -3,9 +3,7 @@ use nu_test_support::playground::Playground;
 use nu_test_support::{nu, nu_repl_code, pipeline};
 use pretty_assertions::assert_eq;
 
-#[cfg(feature = "which-support")]
 mod environment;
-
 mod pipeline;
 mod repl;