feat: add $effect.allowed rune

[Ocean-OS]

This adds the $effect.allowed rune, an advanced feature that indicates whether an effect or async derived can be created in the current context. Effects and async deriveds can only be created when a root effect is active, and previously, the only way to know this was to try creating an effect and checking if it threw an error.
Closes #16366.

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[7nik]

A bit of bikeshading: the name active_root_effect() confuses me. Also, I like Rich's $effect.allowed() name as more close to the rune's purpose.

[Ocean-OS]

Yeah, I couldn't really think of a concise name for active_root_effect
I'm fine with whatever name we choose for the rune (as long as it makes sense), so I'll change it in a bit

[rChaoz]

What about adding a parameter to $effect.tracking to avoid confusion between the 2, like $effect.tracking(true)?

[Ocean-OS]

What would the parameter do?

[rChaoz]

Indicate if currently inside a root effect, rather than a regular effect/derived. So, effect creation is tracked (and hooked up to a lifecycle), but not dependencies.

I hope I didn't misunderstand how this works under the hood, but if correct, the parameter being root: boolean = false makes sense.