[#14968] perf: cache the innermost scope state in ScopedEnvExtension.StateStack

[downstream-lean4]

This is the adaptation PR for leanprover/lean4#14968.

[downstream-lean4]

Build report for downstream: follow upstream PR

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mathlib4	✅	✅ in 1189s	✅ in 49s	🟥 in 94s

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aesop	✅	✅ in 18s	✅ in 5s	⏭️
batteries	✅	✅ in 14s	✅ in 4s	✅ in 2s
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lean4-cli	✅	✅ in 3s	✅ in 0s	⏭️
plausible	✅	✅ in 3s	✅ in 2s	⏭️
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quote4	✅	✅ in 6s	✅ in 1s	⏭️
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comparator		✅ in 3s	⏭️	⏭️
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lean4-unicode-basic		✅ in 4s	⏭️	⏭️
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