BigInt support

[dnicolson]

The object type BigInt is not recognised and keys are created without values resulting in invalid property lists.

Nested BigInteger.js objects are treated as if they are key/value objects and as a result produce dict elements without values and "Value missing for key inside " plutil lint errors.

The following code:

const bigInt = require('big-integer')
const plist = require('plist')

const obj = {key1: bigInt('ffffffffffffffbf', 16), key2: BigInt(1)}
console.log(obj)
console.log(JSON.stringify(obj.key1))
console.log(plist.build(obj))

Produces the following output:

{ key1: Integer { value: 18446744073709551551n }, key2: 1n }
"18446744073709551551"
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>key1</key>
    <dict>
      <key>value</key>
    </dict>
    <key>key2</key>
  </dict>
</plist>