Add support for Western Armenian (hyw)

The usual information is needed: Translatewiki.net languages.

Note that on Wikidata: d:User:Nikki/Permanent duplicates#Armenian, where "hy-arevela" used for Eastern and "hy-arevmda" for Western, because of https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry :

...
%%
Type: variant
Subtag: arevela
Description: Eastern Armenian
Added: 2006-09-18
Prefix: hy
%%
Type: variant
Subtag: arevmda
Description: Western Armenian
Added: 2006-09-18
Prefix: hy
%%
...

Thank you, we are well aware of this subtag which was introduced as an expedient with the knowledge that a formal language code would be required.

All right,

• A language code from ISO 639-3 or the Ethnologue website (for example 'de') - hyw [proposed]
• The language name in English and in its own language (for example 'German – Deutsch (de)') - Western Armenian - արեւմտահայերէն
• Is the language RTL or LTR (written from right to left or left to right) - LTR
• If the language is commonly written in more than one script, please say in which script the language should be enabled. Other variants can be enabled as and when translators for those variants volunteer. - Armn only
• (For MediaWiki) Should the language fall back to another language other than English? If so which? - Fallback should be to hy.

Once the code has been approved, it can be added. For now, this request will be denied.

Afaik the request link: http://www-01.sil.org/iso639-3/chg_detail.asp?id=2017-023&lang=hyw

And now it's approved!

UPDATE: Amire has submitted https://gerrit.wikimedia.org/r/#/c/407155/, should we agree merging it or not?