[locale] ja: Add new 'Reiwa' Era

[eternalharvest]

@caged ichernev

Thank u for merging my PR.
But, since my original PR was written bit long ago, In Japan the era has changed to new 'Reiwa' Era.
So, I need to add new Era definitions for Japanese locale.
This patch fix this issue, and rewrite test cases for this.

I would appreciate if this patch will be merged until next release.
Best, Regards.

Reference (Original PR)

#4599 [feature] Add era support for en,ja

[coveralls]

Coverage increased (+0.0004%) to 88.471% when pulling eb4c395 on eternalharvest:feature/add_new_japanese_reiwa_era into 1021c01 on moment:develop.

[eternalharvest]

sorry, this patch contains some issue related to coding style.
i'll fix this as soon as possible.

[eternalharvest]

I fixed the issue related to the coding style.
Now, It's ready to merge!

[ichernev]

@eternalharvest the most recent 2 eras lack since/until (they only have since), is that on purpose?

[eternalharvest]

@ichernev

Sorry for my late reply.
The reason why the most recent 2 eras lacking until definitions is just totally slipped my mind.
But the result it self is same even if the until definition is set.
Because when the until definition is not defined, the day until is considered as +Infinity and when the period of Eras is overlapped the most recent era is choosed.