process: use frozen array for process.allowedNodeEnvironmentFlags

Maintain no-op calls of Set prototype methods to process.allowedNodeEnvironmentFlags for backward compatibility.

Labeled as semver-major as it makes following code throw:

Set.prototype.add.call(process.allowedNodeEnvironmentFlags, '--user-option`);
process.allowedNodeEnvironmentFlags.has('--user-option') === true;

Originally suggested by @trott in https://github.com/nodejs/node/pull/36660#pullrequestreview-579893872.