Change the autonym of Manipuri to Meetei script

Change the autonym of Manipuri to Meetei script

Meetei Mayek script is the one used in schools in Manipur
(see the Wikipedia article [[Meitei script]] for references).
It is also the script that is used for all the translations
in translatewiki [1] and for all the test articles
in the Wikimedia Incubator.[2]

[1] https://translatewiki.net/w/i.php?title=Special:Translate&filter=translated&group=mediawiki&language=mni&action=translate
[2] https://incubator.wikimedia.org/wiki/Special:PrefixIndex/wp/mni

Corresponding commit in language-data:
https://github.com/wikimedia/language-data/pull/57

Change-Id: I892320f2dfaac05bfc2a544502823a244a9ad1c8