Add BigInt bridging support for Turbo Modules (#56008)

[christophpurrer]

Summary:

This diff adds native BigInt bridging support to React Native's C++ bridging layer.
BigInt is a JavaScript primitive that represents integers with arbitrary precision,
enabling safe handling of 64-bit integers that exceed JavaScript's Number.MAX_SAFE_INTEGER (2^53-1).

This implementation introduces a facebook::react::BigInt C++ wrapper class that
internally stores values as std::variant<int64_t, uint64_t>, preserving signedness.
The wrapper is constructed from jsi::BigInt (checking isInt64()/isUint64() for
lossless conversion) and provides toJSBigInt() for the reverse direction.

A Bridging<BigInt> template specialization converts between jsi::Value and
facebook::react::BigInt, enabling seamless use in Turbo Module method signatures.

Example usage:

// In your Turbo Module
BigInt getBigInt(jsi::Runtime &rt, BigInt arg) {
  return arg;  // Receives BigInt from JS, returns BigInt to JS
}

// In JavaScript
const result = nativeModule.getBigInt(BigInt('9223372036854775807'));

Hey, wait a moment ....

Why are we not simply bridging like this:

struct Bridging<int64_t> {

or

struct Bridging<uint64_t> {

??????

Reason: It is very likely that custom implementations are already present in many RN apps > See: https://reactnative.dev/docs/the-new-architecture/custom-cxx-types

template <>
struct Bridging<int64_t> {
  // Converts from the JS representation to the C++ representation
  static int64_t fromJs(jsi::Runtime &rt, const jsi::String &value) {
    try {
      size_t pos;
      auto str = value.utf8(rt);
      auto num = std::stoll(str, &pos);
      if (pos != str.size()) {
        throw std::invalid_argument("Invalid number"); // don't support alphanumeric strings
      }
      return num;
    } catch (const std::logic_error &e) {
      throw jsi::JSError(rt, e.what());
    }
  }

  // Converts from the C++ representation to the JS representation
  static jsi::String toJs(jsi::Runtime &rt, int64_t value) {
    return bridging::toJs(rt, std::to_string(value));
  }
};

Changelog: [General][Added] - Add BigInt bridging support for Turbo Modules

Differential Revision: D95706781

[meta-codesync]

@christophpurrer has exported this pull request. If you are a Meta employee, you can view the originating Diff in D95706781.