Add a number guessing version

[nbgraham]

Like https://nerdlegame.com/

Maybe use some code like this to generate the answer

const ops = '-+*/='
const nums = '0123456789'
const desiredLength = 8;

function getOne() {
  let one;
  while (!one) {
    one = tryOnce();
  }
  return one;
}

function tryOnce() {
  let current = '';

  while (current.length < desiredLength) {
    const prev = current[current.length - 1];
    const nextCanBeOp = nums.includes(prev);
    const nextCanBeNum = prev !== '0';

    let choices;
    if (nextCanBeOp && nextCanBeNum) {
      choices = Math.random() < 0.5 ? nums : ops; // 50/50 chance of choosing a number or operation next
    } else if (nextCanBeOp) {
      choices = ops;
    } else if (nextCanBeNum) {
      choices = nums;
    } else {
        return;
    }

    const index = Math.floor(Math.random() * choices.length);
    const char = choices[index];

    if (char === '=') {
      const result = eval(current);
      if (result >= 0) { // require the answer to be a single non-negative number
        const nerdle = current + '=' + Math.floor(result).toString();
        if (nerdle.length === desiredLength) {
          return nerdle;
        }
      }
      return undefined;
    } else {
      current += char;
    }
  }
}