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.

crates/nu-command/tests/commands/mod.rs

@@ -125,7 +125,6 @@ mod upsert;
 mod url;
 mod use_;
 mod where_;
-#[cfg(feature = "which-support")]
 mod which;
 mod while_;
 mod with_env;