Don't add expansion error offset twice

Like the $status commit, this would add the offset to already existing
errors, so

```fish
(foo)
(bar)

something
```

would see the "(foo)" error, store the correct error location, then
see the "(bar)" error, and *add the offset of (bar)* to the "(foo)"
error location.

Solve this by making a new error list and appending it to the existing
ones.

There's a few other ways to solve this, including:

- Stopping after the first error (we only display the first anyway, I
think?)
- Making it so the source location has an "absolute" flag that shows
the offset has already been added (but do we ever need to add two offsets?)

I went with the simpler fix.

src/parse_util.cpp

@@ -1118,13 +1118,14 @@ static bool detect_errors_in_decorated_statement(const wcstring &buff_src,
 
     const wcstring &unexp_command = dst.command.source(buff_src, storage);
     if (!unexp_command.empty()) {
-        wcstring command;
         // Check that we can expand the command.
+        // Make a new error list so we can fix the offset for just those, then append later.
+        wcstring command;
+        parse_error_list_t new_errors;
         if (expand_to_command_and_args(unexp_command, operation_context_t::empty(), &command,
-                                       nullptr, parse_errors,
+                                       nullptr, &new_errors,
                                        true /* skip wildcards */) == expand_result_t::error) {
             errored = true;
-            parse_error_offset_source_start(parse_errors, source_start);
         }
 
         // Check that pipes are sound.
@@ -1174,6 +1175,11 @@ static bool detect_errors_in_decorated_statement(const wcstring &buff_src,
                                               unexp_command.c_str());
             }
         }
+
+        if (parse_errors) {
+            parse_error_offset_source_start(&new_errors, source_start);
+            vec_append(*parse_errors, std::move(new_errors));
+        }
     }
     return errored;
 }

tests/checks/syntax-error-location.fish

@@ -10,3 +10,15 @@ $status
 # CHECK: <fish: The 'exec' command can not be used in a pipeline>
 # CHECK: <echo foo | exec grep # this exec is not allowed!>
 # CHECK: <           ^>
+
+echo '
+
+(true one)
+(true two)
+
+# more things
+' | $fish 2>| string replace -r '(.*)' '<$1>'
+
+# CHECK: <fish: Command substitutions not allowed>
+# CHECK: <(true one)>
+# CHECK: <^>