perf: store current_sources as Set for O(1) membership checks (#18278)

`current_sources` tracks the sources created within the active reaction
so that reading or writing them during that same reaction doesn't
trigger a re-run. It was an `Array` checked with
`Array.prototype.includes.call(...)` in three hot places: the
`state_unsafe_mutation` guard, `set()`'s destruction check, and
`schedule_possible_effect_self_invalidation`.

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Co-authored-by: Rich Harris <rich.harris@vercel.com>