getValueForExpr: add support for arrays

[gilbox]

getValueForExpr: add support for arrays

Solves #10613 by adding support for array indexes for the items attribute of amp-list.

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[googlebot]

CLAs look good, thanks!

[robinpokorny]

I find this unnecesary complicated, it would be sufficient to use (maybe better formated and/or with variables):

- if (!isObject(value) ||
+ if (!(isObject(value) || isArray(value))||
            value[part] === undefined ||
            !hasOwnProperty(value, part)) {
      value = undefined;
      break;
    }

The flow is harder to follow with the added continues. And arrayHasKey feels really hacky: hasOwnProperty should replace that magic in the first part of it and the second part needs no typeof.

[jridgewell]

We could likely just remove this in favor of a truthy check:

if (value && value[part] !== undefined && hasOwnProperty(value, part)) {
  //...
}

[gilbox]

great feedback y'all! I've added a commit that I hope addresses all of your comments

[cschubiner]

Exciting stuff, thanks @gilbox!

[ampprojectbot]

This issue seems to be in Pending Triage for awhile. Please triage this to an appropriate milestone.

[jridgewell]

Apologies for letting this fall through the cracks.