package.json: allow additional properties for alternateTools

In https://github.com/golang/vscode-go/pull/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 golang/vscode-go#526

Change-Id: I72d2a7e72702d13bc74ed72a34a93756569bb041
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/248619
Reviewed-by: Suzy Mueller <suzmue@golang.org>
diff --git a/package.json b/package.json
index f94b2ae..4ea6b76 100644
--- a/package.json
+++ b/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
         }
       }
     },