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

gulpfile.js

@@ -1742,7 +1742,7 @@ gulp.task("lint", function (done) {
   const esLintOptions = [
     "node_modules/eslint/bin/eslint",
     "--ext",
-    ".js,.jsm",
+    ".js,.jsm,.json",
     ".",
     "--report-unused-disable-directives",
   ];