Add line comment language configuration for go.mod (#2761)

languages/go.mod.language-configuration.json

@@ -0,0 +1,5 @@
+{
+	"comments": {
+		"lineComment": "//"
+	}
+}

package.json

@@ -90,7 +90,8 @@
         ],
         "aliases": [
           "Go Module File"
-        ]
+        ],
+        "configuration": "./languages/go.mod.language-configuration.json"
       },
       {
         "id": "go.sum",