Add BigInt

[vieira]

Add syntax highlighting to BigInt literals and to the BigInt built-in object as defined in tc39/proposal-bigint.

const prevMaxSafe = BigInt(Number.MAX_SAFE_INTEGER);
const maxPlusOne = prevMaxSafe + 1n;

const zero = 0n;
const max = 2n ** (64n - 1n) - 1n;
const one = 0b1n;
const hex = 0xFn;