Added Elixir to major modes docs

[tgroshon]

Update the docs to add Elixir as a major mode!

Works in tandem with VSpaceCode change VSpaceCode/VSpaceCode#344.

Updates:

• Default Keybindings doc with the new Elixir major mode bindings
• Major Modes doc with required Elixir LS extension

[stevenguh]

Thanks for the update. I removed the key bindings update. That page will automatically update when we release :)

[tgroshon]

I removed the key bindings update. That page will automatically update when we release :)

Very cool! Thanks for handling that.