Conditional enabling BigInt API with NAPI_EXPERIMENTAL

[legendecas]

Strictly aligning with Node.js header js_native_api.h in which
napi_bigint related apis were nested in NAPI_EXPERIMENTAL.

[NickNaso]

It's ok, but I think that we need to do the same think in all the test code.

[mhdawson]

This may change the ways that the testing works.

This is in js_native_api.h

#define NAPI_VERSION_EXPERIMENTAL 2147483647
#ifndef NAPI_VERSION
#ifdef NAPI_EXPERIMENTAL
#define NAPI_VERSION NAPI_VERSION_EXPERIMENTAL
#else
// The baseline version for N-API.
// The NAPI_VERSION controls which version will be used by default when
// compilling a native addon. If the addon developer specifically wants to use
// functions available in a new version of N-API that is not yet ported in all
// LTS versions, they can set NAPI_VERSION knowing that they have specifically
// depended on that version.
#define NAPI_VERSION 5
#endif
#endif

I believe the CI jobs for the node-addon-api set NAPI_VERSION.

So using the define versus comparing againts NAPI_VERSION might result in less testing or failed tests. We'd need to run against all of the versions and double check we get the same tests run.

[legendecas]

Closed in favor of #545.