Add basic linting of JSON files using eslint-plugin-json

By adding basic linting of JSON files, we can ensure that they're actually valid and prevent e.g. test-failures caused by *accidental* errors when editing the `test/test_manifest.json` file (something that I've done *many* times myself).

For now this simply uses the `recommended` configuration, but we can obviously tweak this later if/when needed. Please find additional information at https://github.com/azeemba/eslint-plugin-json

test/types/tsconfig.json

@@ -22,5 +22,5 @@
   },
   "files": [
     "main.ts"
-  ],
+  ]
 }