Allow parsing scalars and mappings into the same type

Some YAML schemas seem to support specifying values either as a single
string, or as a dictionary.

One practical example is the `image` option in `.gitlab-ci.yml` files:

https://docs.gitlab.com/ee/ci/yaml/#image

GitLab accepts either

```yaml
test-job:
  image: ruby:3.0
```

or

```yaml
test-job:
  image:
    name: ruby:3.0
```

This change enables us to support this functionality, by calling
`fromString` only on scalar nodes; mappings are deserialized as usual.

source/configy/read.d

@@ -718,8 +718,15 @@ package FR.Type parseField (alias FR)
     }
 
     else static if (hasFromString!(FR.Type))
-        return wrapConstruct(FR.Type.fromString(node.as!string), path, Location.get(node));
-
+    {
+        if (node.nodeID == NodeID.mapping)
+            return node.parseMapping!(FR)(path, defaultValue, ctx, null);
+        else
+        if (node.nodeID == NodeID.scalar)
+            return wrapConstruct(FR.Type.fromString(node.as!string), path, Location.get(node));
+        else
+            throw new TypeConfigException(node, "a mapping (object) or a scalar", path);
+    }
     else static if (hasStringCtor!(FR.Type))
         return wrapConstruct(FR.Type(node.as!string), path, Location.get(node));
 

source/configy/test.d

@@ -818,6 +818,36 @@ unittest
     assert(v2.v2.str == "hello world");
 }
 
+/// Test calling fromString on scalars only
+unittest
+{
+    static struct Job {
+        static struct Image {
+            string name;
+            static Image fromString(string s) {
+                Image v;
+                v.name = s;
+                return v;
+            }
+        }
+        Image image;
+    }
+    // alias Config = Job[string]; // TODO, we should support top-level associative arrays
+    struct Config { Job job; }
+
+    auto c1 = parseConfigString!Config(`
+job:
+  image: ruby:3.0
+`, "/dev/null");
+    auto c2 = parseConfigString!Config(`
+job:
+  image:
+    name: ruby:3.0
+`, "/dev/null");
+    assert(c1.job.image.name == "ruby:3.0");
+    assert(c2.job.image.name == "ruby:3.0");
+}
+
 /// Don't call `opCmp` / `opEquals` as they might not be CTFEable
 /// Also various tests around static arrays
 unittest