Add mw.site.interwikiMap

This makes the interwiki map available to Lua modules. The code is
based on the API interwiki map code in core (the appendInterwikiMap
method of includes/api/ApiQuerySiteInfo.php.) Everything that the
API includes is added, apart from iw_api and iw_wikiid, which I
couldn't think of a use for from Lua modules.

Accessing the interwiki map would be useful for modules like
enwiki's Module:InterwikiTable,[1] as it would stop module writers
having to duplicate the data.

[1] https://en.wikipedia.org/wiki/Module:InterwikiTable

Change-Id: Ie8ad2582aaf5e422824f7da51714a347bb4041d1