This implements storing named captures on the regexp result object.
For instance, /(?<a>.)/u.exec("b") will return a result such that:

result.group.a  // "b"

The spec proposal is not yet final, so this may still change in the future.

BUG=v8:5437

Review-Url: https://codereview.chromium.org/2630233003
Cr-Commit-Position: refs/heads/master@{#42532}