fix: move bignum-dependent multipleOf tests to optional/bignum.json

[Sagar-6203620715]

What this changes

Two test groups in the core multipleOf.json files require arbitrary-precision arithmetic to evaluate correctly, making them de-facto mandatory bignum tests. This violates the suite's own convention that bignum support is optional.

The two affected test groups, moved across all 6 drafts:

• "float division = inf" — validates 1e308 against multipleOf: 0.123456789, expects valid: false. Assumes implementations overflow to false rather than erroring/throwing, which is not guaranteed on all platforms.

• "small multiple of large integer" — validates 12391239123 against multipleOf: 1e-8, expects valid: true. Requires arbitrary-precision arithmetic to compute correctly; IEEE 754 double implementations will silently produce a wrong result.

Files modified

Each multipleOf.json retains exactly 3 test groups: "by int", "by number", and "by small number" — none of which require bignum support.

Notes

• Draft-specific $schema URIs are preserved exactly as they appeared in the original multipleOf.json files (draft4/6/7 have no $schema; 2019-09, 2020-12, and v1 retain their respective URIs).
• The wording difference in draft4 ("invalid, but naive..." vs "always invalid, but naive..." in later drafts) is intentionally preserved.
• All 12 modified files pass JSON validation.
• No other tests are affected.

Fixes #536

[Sagar-6203620715]

/cc @Julian