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_syntax/tests/snapshots/pass/pizza_dbg.expr.formatted.roc

@@ -0,0 +1 @@
+1 |> dbg

crates/compiler/test_syntax/tests/snapshots/pass/pizza_dbg.expr.result-ast

@@ -0,0 +1,16 @@
+SpaceAfter(
+    BinOps(
+        [
+            (
+                @0-1 Num(
+                    "1",
+                ),
+                @2-4 Pizza,
+            ),
+        ],
+        @5-8 Dbg,
+    ),
+    [
+        Newline,
+    ],
+)

crates/compiler/test_syntax/tests/snapshots/pass/pizza_dbg.expr.roc

@@ -0,0 +1 @@
+1 |> dbg

crates/compiler/test_syntax/tests/test_snapshots.rs

@@ -442,6 +442,7 @@ mod test_snapshots {
         pass/pattern_as_spaces.expr,
         pass/pattern_with_space_in_parens.expr, // https://github.com/roc-lang/roc/issues/929
         pass/pizza_bang.moduledefs,
+        pass/pizza_dbg.expr,
         pass/plus_if.expr,
         pass/plus_when.expr,
         pass/pos_inf_float.expr,