feat: add m2m in dimension file

Author: valentinemilov

sml-reference/dimension.md

@@ -185,6 +185,7 @@ namespace Dimensions{
       Object to
       String role_play
       String type
+      Boolean m2m
     }
     class From{
       String dataset
@@ -508,6 +509,13 @@ marks):
 For example, if you wanted to use the prefix **Order**, you would set
 `role_play` to `"Order {0}"`.
 
+## m2m
+
+- **Type:** boolean
+- **Required:** N
+
+Defines a many-to-many relationship. The property is supported only for embedded relationships.
+
 # Calculation Group Properties
 
 ## unique_name