Add vscode support for Elixir

[venkatd]

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

This PR adds support for the Elixir language on vscode. Screenshot of it in action below:

Thanks to @rkolpakov for the bulk of the work.

[venkatd]

@JakeDawkins sorry to bother you, anything we can do to ease the process for getting this merged in? Our team is currently using a custom build of vscode-apollo to get Elixir support. Thanks!