Conform bigint literals to the new ESTree spec

[bradzacher]

For this code example:

const x = 1n;

We currently emit the following AST (excluding the variable decl):

{
  "type": "BigIntLiteral",
  "raw": "1n",
  "value": "1",
}

When we should be emitting this (as per the ESTree spec

{
  "type": "Literal",
  "raw": "1n",
  "value": 1n,
  "bigint": "1"
}