fix: accept start as keyword arg in enumerate builtin

[MukundaKatta]

Closes #394.

The enumerate(iterable, start) shim only accepted start positionally, but stdlib enumerate allows start as a keyword (enumerate(iter, start=0)). Switched the parser to walk kwargs so start= works as a keyword while preserving the existing positional and arity errors, mirroring the kwarg-handling pattern used in zip and sorted.

Test added in crates/monty/test_cases/builtin__more_iter_funcs.py covering the issue's reproducer and an unknown-kwarg rejection.

[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks
⏩ 15 skipped benchmarks¹

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_{Comparing MukundaKatta:fix/enumerate-keyword-start (92aa59d) with main (33e47c6)}

Footnotes

1. 15 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[devin-ai-integration]

Devin Review found 1 potential issue.

View 3 additional findings in Devin Review.

[devin-ai-integration]

🔴 Reference count leak / panic: start not protected by defer_drop! before fallible MontyIter::new call

If MontyIter::new(iterable, vm)? fails (e.g. enumerate(42, [1, 2, 3]) where 42 is not iterable), the ? operator causes an early return before defer_drop!(start, vm) on line 27 is reached. When start holds a Value::Ref (heap-allocated), this means drop_with_heap is never called — with the memory-model-checks feature (used in CI), this panics; without it, it silently leaks the reference count.

The correct pattern — used by analogous builtins sum (crates/monty/src/builtins/sum.rs:23) and filter (crates/monty/src/builtins/filter.rs:35) — is to register the secondary value with defer_drop! before the fallible MontyIter::new call.

Suggested change

	let (iterable, start) = parse_enumerate_args(args, vm)?;
	let iter = MontyIter::new(iterable, vm)?;
	defer_drop_mut!(iter, vm);
	defer_drop!(start, vm);
	let (iterable, start) = parse_enumerate_args(args, vm)?;
	defer_drop!(start, vm);
	let iter = MontyIter::new(iterable, vm)?;
	defer_drop_mut!(iter, vm);

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