Avoid IIFE wrapping in IR.Abs lowering - #170

IR.Abs lowering was routing every body through asExpression, so a Let or IfThenElse body became (function() chunk end)() inside a return. removeScopeWhenInsideEmptyFunction then had to undo that on every such lambda.

Changes:

• lib/Language/PureScript/Backend/Lua.hs — IR.Abs now uses the chunk-aware go path. When the body lowers to Left chunk, it emits Lua.functionDef luaParams chunk directly; only Right expr falls back to [Lua.return e].
• lib/Language/PureScript/Backend/Lua/Optimizer.hs — removed the now-dead removeScopeWhenInsideEmptyFunction rule and its Return import.
• test/Language/PureScript/Backend/Lua/Optimizer/Spec.hs — removed the test for the deleted rule.

Before:

function(x)
  return (function()
    local y = f(x)
    return y
  end)()
end

After:

function(x)
  local y = f(x)
  return y
end

Summary

Checklist

• Added a changelog.d/ fragment for any user-facing change (scriv create
  in the dev shell), or this change ships nothing releasable (CI, docs, or an
  internal refactor).
• In the dev shell (nix develop), fourmolu -i lib/ exe/ test/ and
  hlint lib/ exe/ test/ are clean.
• In the dev shell, cabal test all passes; structural goldens were
  re-accepted on purpose if codegen moved (PSLUA_GOLDEN_ACCEPT=1), and
  eval/golden.txt still holds.

Fixes #158.

Supersedes #160: same commits, reopened as a maintainer-authored PR so the required workflow runs are created (runs on PRs authored by the Copilot agent require manual approval and were never created for the follow-up pushes).