Support passing values into dbg with the pipe operator

In order to desugar `dbg` in a pipeline we need to allow a bare `dbg`
node in desugaring and only report it as an error if the bare node
survives to the next step of canonicalization. This means we move the
error code out of `desugar_expr` and into `canonicalize_expr`. This is
much simpler to do now that these functions use the same `env` struct,
since previously we would have had to pass down extra args to
`canonicalize_expr`. Sharing the `env` struct means that we also don't
have to worry about calculating `line_info` more than once.

crates/compiler/test_mono/src/tests.rs

@@ -3295,6 +3295,18 @@ fn dbg_inside_string() {
     )
 }
 
+#[mono_test]
+fn pizza_dbg() {
+    indoc!(
+        r#"
+        1
+        |> dbg
+        |> Num.add 2
+        |> dbg
+        "#
+    )
+}
+
 #[mono_test]
 fn linked_list_reverse() {
     indoc!(