add Crystal support

[mamantoha]

Requirements (please check off with 'x')

• [x] I have read the Contributing Guidelines
• [x] I have read or at least glanced at the FAQ
• [x] I have read or at least glanced at the Wiki

What does this Pull Request (PR) do?

Add icon for Crystal programming language

How should this be manually tested?

Add to your ~/.vimrc

let g:WebDevIconsUnicodeDecorateFileNodesExtensionSymbols['cr'] = ''

Any background context you can provide?

Nerd Fonts 2.2.0-RC is required

What are the relevant tickets (if any)?

https://github.com/ryanoasis/nerd-fonts/pull/429

Screenshots (if appropriate or helpful)