fix: Correctly detect Node environment

[felixrieseberg]

Right now, v86 prefers XMLHttpRequest when available. I'm patching this in my Electron use case to prefer fs for performance reasons.

With this change, v86 prefers XMLHttpRequest when available and process.versions.node is undefined. If desired, I'm also happy to write a simple try/catch function that returns a boolean if require('fs') works.

[copy]

With this change, v86 prefers XMLHttpRequest when available and process.versions.node is undefined. If desired, I'm also happy to write a simple try/catch function that returns a boolean if require('fs') works.

I think it would be nice if we could move all three occurrences into src/lib.js and write this only once, e.g.:

let fs;
if(typeof require !== "undefined")
{
    try {
        fs = require("fs");
    }
    catch(e) {}
}

if(fs)
{
    v86util.determine_size = ... // using fs
    v86util.load_file = ...
}
else
{
    v86util.determine_size = ... // using xhr
    v86util.load_file = ...
}