[misc] Add 'ss' relative time key to typescript definition

[xkxx]

Hi there,

This patch adds 'ss' to RelativeTimeKey to make the typescript definition more complete, and updates the typing tests to match.

First patch to moment.js, so please let me know if there's anything I'm missing or if you have any suggestions for improvements!

[ichernev]

This is the same as #4138 but slightly better, maybe :)

[xkxx]

Hi ichernev!

Is there anything that I need to address in the PR for it to be accepted?

Thanks,

xkxx

[ichernev]

@xkxx don't you also need https://github.com/moment/moment/pull/4138/files#diff-3dd6cebe542b0e207494192c4bbf4a22R217

[jsf-clabot]

All committers have signed the CLA.

[xkxx]

Hi @ichernev,

How's the PR looking now? (Please ignore the git author screwup earlier :/)

[xkxx]

Hello @ichernev again,

Could you take a look at this PR and see if there's anything else for it to be merged?

Thanks,

xkxx

[xkxx]

Hello, just a friendly ping if anyone would like to take another look at the PR :)

[butterflyhug]

I think this looks good -- tagging for inclusion in the next release.