fix(schema): use big.js to avoid floating-point errors in multipleOf

[spiderocious]

Summary

Fixes multipleOf validation incorrectly rejecting valid numbers due to floating-point rounding errors. For example, 100 with multipleOf: 0.01 was reported as invalid because 100 % 0.01 evaluates to 0.0099... rather than 0 in plain JavaScript arithmetic.

As suggested by @lukad in the issue, this replaces the native modulo check with arbitrary-precision arithmetic via big.js, which eliminates this class of rounding error entirely. big.js adds ~3KB
(minified + gzipped) and has zero dependencies.

Changes Made

• src/validation/number.ts

  • Replaced the naive value % multipleOf !== 0 check with an isMultipleOf helper that uses big.js for the modulo operation.
  • The helper guards against the RangeError big.js throws when an operation exceeds its precision limit (e.g. 1e308 divided by a tiny multipleOf); in that case the value is treated as not a multiple, matching the expected invalid result instead of throwing.

• test/json-schema-test-suite/failed-json-schema-test-suite.json

  • Re-enabled two official JSON Schema Test Suite cases that were previously ignored because of this bug: "0.0075 is multiple of 0.0001" and "any integer is a multiple of 1e-8". They now pass.

• test/validation/number.test.ts

  • Added a unit test covering floating-point cases (incl. the issue's 100 / 0.01 example) and a negative case (0.00751 is not a multiple of 0.0001) to confirm genuine non-multiples still fail.

• package.json

  • Added big.js (dependency) and @types/big.js (devDependency).

Related Issues/PRs

Fixes #222

Screenshots (Before/After if applicable)

N/A — validation-only change.

Checklist

• I have read the Contributing Guidelines and followed them.
• Unit tests were added.