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"

https://tc39.github.io/proposal-regexp-named-groups/

BUG=v8:5437

Review-Url: https://codereview.chromium.org/2630233003
Cr-Original-Original-Original-Commit-Position: refs/heads/master@{#42532}
Committed: https://chromium.googlesource.com/v8/v8/+/70000946eb2a9155679528702a766219a1fcf154
Review-Url: https://codereview.chromium.org/2630233003
Cr-Original-Original-Commit-Position: refs/heads/master@{#42570}
Committed: https://chromium.googlesource.com/v8/v8/+/ee94fa11ed63477393c6534c352ebac6e502396c
Review-Url: https://codereview.chromium.org/2630233003
Cr-Original-Commit-Position: refs/heads/master@{#42676}
Committed: https://chromium.googlesource.com/v8/v8/+/8bf52534f6bf86821a1589dcbcb7335052c1f94f
Review-Url: https://codereview.chromium.org/2630233003
Cr-Commit-Position: refs/heads/master@{#42838}