Use correct language name for Elixir

[sheharyarn]

Currently, the syntax highlighting code for the Elixir language uses ex not elixir which is incorrect, has caused a lot of confusion and does not implement highlighting for the language in almost 100% of the cases where the language is inferred from the name/tag.

The biggest example is Stackoverflow itself:

• SE feature request was opened back in 2013
• Elixir support was added in 2017
• But almost zero of the Elixir code on the website is automatically highlighted
• To highlight manually, we have to set the language to ex; which again, is incorrect

Renaming this to lang-elixir will fix the syntax highlighting issue through out the ecosystem.

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[sheharyarn]

I signed it!

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[sheharyarn]

Any update on this?