perf(quickcheck): build arbitrary strings linearly

[mizchi]

Build arbitrary strings with StringBuilder instead of repeatedly concatenating one character at a time. This changes construction from quadratic copying to a linear append loop while preserving random-state consumption and generated values.

Native benchmark (size=10000, 100 generated strings):

before	after	change
84.85 ms	3.26 ms	96% faster

Validation: all 82 QuickCheck tests pass on wasm, wasm-gc, JS, and native; moon check --target all and moon info pass.

[Copilot]

Pull request overview

Improves QuickCheck’s Arbitrary implementation for String by switching from repeated string concatenation (quadratic copying) to a StringBuilder append loop (linear construction), and adds a benchmark test to track the performance win.

Changes:

• Refactor Arbitrary for String to build via StringBuilder.write_char and a final to_string().
• Add a dedicated benchmark test for generating 100 strings at size 10,000.
• Add moonbitlang/core/bench as a test-only dependency for the quickcheck package.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
quickcheck/moon.pkg	Adds bench as a test-only import to support the new benchmark test.
quickcheck/arbitrary.mbt	Replaces per-character string concatenation with StringBuilder to make generation linear-time.
quickcheck/arbitrary_string_bench_test.mbt	Introduces a benchmark test to measure Arbitrary String generation performance.

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