feat: add OrderedMap, the table that keeps its key order

Assisted-by: GLM 5.3 Flash

document.go

@@ -20,8 +20,13 @@ type Document struct {
 	footer []string
 }
 
-// Root returns the document's root table.
-func (d *Document) Root() *Table { return d.root }
+// Root returns the document's root table. A nil document has no root.
+func (d *Document) Root() *Table {
+	if d == nil {
+		return nil
+	}
+	return d.root
+}
 
 // Map returns the value tree, the shape ParseMap gives. It is the tree the
 // document was parsed into, not a copy.