added include statement

Signed-off-by: Jiaxiao Zhou <jiazho@microsoft.com>

Author: Mossaka

design/mvp/WIT.md

@@ -625,21 +625,22 @@ bare as an identifier. These are used to help parse the format, and the list of
 keywords is still in flux at this time but the current set is:
 
 ```ebnf
-keyword ::= 'use'
-          | 'type'
-          | 'resource'
-          | 'func'
-          | 'record'
+keyword ::= 'default'
           | 'enum'
+          | 'export'
           | 'flags'
-          | 'variant'
-          | 'union'
-          | 'static'
+          | 'func'
+          | 'import'
+          | 'include'
           | 'interface'
+          | 'record'
+          | 'resource'
+          | 'static'
+          | 'type'
+          | 'union'
+          | 'use'
+          | 'variant'
           | 'world'
-          | 'import'
-          | 'export'
-          | 'default'
 ```
 
 ## Top-level items
@@ -662,7 +663,7 @@ Concretely, the structure of a world is:
 ```ebnf
 world-item ::= 'default'? 'world' id '{' world-items* '}'
 
-world-items ::= export-item | import-item | use-item | typedef-item
+world-items ::= export-item | import-item | use-item | typedef-item | include-item
 
 export-item ::= 'export' id ':' extern-type
 import-item ::= 'import' id ':' extern-type
@@ -676,6 +677,25 @@ interface-type ::= 'interface' '{' interface-items* '}'
 Note that worlds can import types and define their own types to be exported
 from the root of a component and used within functions imported and exported.
 
+## Item: `include`
+
+A `include` statement enables union the current world with another world. The structure of an `include` statement is:
+
+```wit
+include pkg.my-world-1 with { a as a1, b as b1 }
+include self.my-world-2
+```
+
+```ebnf
+include-item ::= 'include' use-path
+               | 'include' use-path 'with' '{' include-names-list '}'
+
+include-names-list ::= include-names-item
+                 | include-names-item ',' include-names-item?
+
+include-names-item ::= id 'as' id
+```
+
 ## Item: `interface`
 
 Interfaces can be defined in a `wit` document. Interfaces have a name and a