Consider using standard format for `amp-timeago` locale

[TomerAberbach]

Describe the new feature or change to an existing feature you'd like to see

I would like amp-timeago's locale attribute to use some standardized format, such as IETF language tags (e.g. zh-CN), as opposed to some custom format (zhCN).

Describe alternatives you've considered

I've formatted locales to amp-timeago's format in the meantime.

Additional context

Using a non-standard format causes unnecessary headaches for developers because the default locale libraries of programming languages (e.g. Java) do not support the format out of the box. They support the standard format (and output it).

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