fix: use BigInt for large integers to preserve precision

[remorses]

Fixes #29

Numbers larger than Number.MAX_SAFE_INTEGER (2^53-1) lose precision when converted to JavaScript Number.

// Before: precision lost
mri(['--id', '9007199254740993'])
// { id: 9007199254740992 }

// After: BigInt preserves precision  
mri(['--id', '9007199254740993'])
// { id: 9007199254740993n }

This affects downstream packages like cac which uses mri for argument parsing.

Behavior:

• Safe integers (≤ 9007199254740991) → number
• Large integers (> MAX_SAFE_INTEGER) → bigint
• Floats, scientific notation, leading zeros → number (unchanged)

Tests added:

• Large integers use BigInt
• MAX_SAFE_INTEGER boundary
• MIN_SAFE_INTEGER boundary
• Large negative integers
• Leading zeros remain numbers

cc @lukeed

[Copilot]

Pull request overview

This PR fixes precision loss for large integers in command-line argument parsing by automatically using BigInt for integer strings that exceed JavaScript's safe integer range (2^53-1). The implementation adds a new toNum function that detects when numeric conversion would lose precision and uses BigInt instead.

Changes:

• Added toNum function to detect and preserve precision for large integers using BigInt
• Refactored numeric conversion logic in toVal to use the new toNum function
• Added comprehensive tests for BigInt boundary conditions and negative large integers

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/index.js	Implements new toNum function with BigInt support for precision-loss detection and integrates it into the value conversion logic
test/num.js	Adds three test cases covering large integer BigInt conversion, MAX_SAFE_INTEGER boundary behavior, and large negative integers

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[lukeed]

This is technically a breaking change, as bigints werent supported in Node until 10.x and this lib is still 4+
And I was considering dropping number-parsing altogether in the next major.

While this PR is technically correct, I wonder how frequent it's being bumped into

Thoughts?

[remorses]

I can update the PR to return a number if BigInt constructor is missing. I don't think making a breaking change for this issue is worth it. Number parsing is a fine in my opinion