Add elixir as a language option

[ThatHurleyGuy]

I saw that elixir was now supported in the leetcode web UI, and thought it would be nice to be able to do that within vscode. This PR basically just adds elixir anywhere else that has boilerplate for other languages. I wasn't sure the best way to test this, so I deleted ~/.vscode/extensions/leetcode.vscode-leetcode-0.18.0/out and ~/.vscode/extensions/leetcode.vscode-leetcode-0.18.0/node_modules/vsc-leetcode-cli and replaced the contents with my forked repos. After that I was able to set my default language and generate/test/submit problems all in elixir.

This work will depend on an update the the cli repo dependency. See associated PR here. So we'll need to update that dependency once the other PR is merged/released. If there's anything missing from this PR or if there's anything else I can provide to help with this!