package.json: allow additional properties for alternateTools

In #284, we started
to use additionalProperties to support vscode's new setting UI
feature available for object type settings if the settings
accept only predefined properties.

 "go.alternateTools" can accept more tool names than those
specified in package.json. So, correct its additionalProperties
value.

Fixes #526

Change-Id: I72d2a7e72702d13bc74ed72a34a93756569bb041
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/248619
Reviewed-by: Suzy Mueller <[email protected]>

package.json

@@ -1927,7 +1927,7 @@
               "description": "Alternate tool to use instead of the guru binary or alternate path to use for the guru binary."
             }
           },
-          "additionalProperties": false
+          "additionalProperties": true
         }
       }
     },