Add return type to function declarations

src/grammar.json

@@ -1277,6 +1277,31 @@
             "type": "SYMBOL",
             "name": "parameters"
           }
+        },
+        {
+          "type": "CHOICE",
+          "members": [
+            {
+              "type": "SEQ",
+              "members": [
+                {
+                  "type": "STRING",
+                  "value": ":"
+                },
+                {
+                  "type": "FIELD",
+                  "name": "return_type",
+                  "content": {
+                    "type": "SYMBOL",
+                    "name": "identifier"
+                  }
+                }
+              ]
+            },
+            {
+              "type": "BLANK"
+            }
+          ]
         }
       ]
     },

src/node-types.json

@@ -1012,6 +1012,16 @@
             "named": true
           }
         ]
+      },
+      "return_type": {
+        "multiple": false,
+        "required": false,
+        "types": [
+          {
+            "type": "identifier",
+            "named": true
+          }
+        ]
       }
     }
   },
@@ -1452,6 +1462,16 @@
             "named": true
           }
         ]
+      },
+      "return_type": {
+        "multiple": false,
+        "required": false,
+        "types": [
+          {
+            "type": "identifier",
+            "named": true
+          }
+        ]
       }
     }
   },