Gpkg layers can have extra, but now restrict the name pattern (eg, no ' ')

Author: chbrandt

json_schema/geopackage_layers.schema.json

@@ -24,5 +24,9 @@
     "surface_features"
   ],
 
-  "additionalProperties": true
+  "patternProperties" : {
+    "^[A-Za-z][A-Za-z0-9_]*$" : { "type" : "string" }
+  },
+
+  "additionalProperties" : { "type" : "string" }
 }