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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Preview: https://svelte-dev-git-preview-svelte-16458-svelte.vercel.app/

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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