process: use frozen array for process.allowedNodeEnvironmentFlags

[aduh95]

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 #36660 (review).

[benjamingr]

I don't like having a weird set-like-array-object around - wouldn't it be simpler to subclass SafeSet in a way that noops on mutations?

[benjamingr]

Oh, I see that's what the current code does, then the issue is getting external set prototype calls on the set mutating it?

[aduh95]

Yes, I did a proposal in #36660 to circumvent the direct Set prototype calls using subclassing, and the suggestion from Rich was to use a frozen object instead (this PR).

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