Update table schema

When exporting a diagram that contains a table with indices to JSON then importing it, an validation error occurs because the schema for the index on the table was not correctly setup leading to the file being considered an invalid json.

src/data/schemas.js

@@ -47,7 +47,12 @@ export const tableSchema = {
           unique: { type: "boolean" },
           fields: {
             type: "array",
-            items: { type: "string" },
+            items: { 
+                type: "object", properties: {
+                  type: { type: "string" },
+                  value: { type: "string" }
+                }
+            },
           },
         },
         required: ["name", "unique", "fields"],