Fix parsing flex-basis in shorthand flex when not a value

[matmannion]

Fixes the case in the flex shorthand where the third value isn't an explicit value or dimension, which was causing a parse error when called with (e.g.) flex: 0 0 auto

[domenic]

@asamuzaK can you take a look?

@matmannion would you be willing to send a regression test to jsdom as well? Similar to, e.g., jsdom/jsdom#3893

[asamuzaK]

We also need to fix L56 block to:

if (name === "none") {
  return {
    "flex-grow": "0",
    "flex-shrink": "0",
    "flex-basis": "auto"
  };
}
flex["flex-basis"] = name;
return flex;

And L126 block to:

case "Identifier": {
  flex["flex-basis"] = val2.name;
  break;
}

[matmannion]

Thanks @asamuzaK, have updated.

@domenic no problem, I'll PR adding the jsdom regression test

[asamuzaK]

LGTM

[heath-freenome]

Sweet! When can we get this released?

[domenic]

jsdom releases usually happen on weekends when I am free (which is not every weekend).