Add syntax highlighting support for Swift

[jaclync]

Based on the initial work by @astralbodies #165

Regex and JavaScript aren't my strong suits, please feel free to correct/suggest anything - I just really wanted to see Swift code in p2 😄

Changes proposed in this Pull Request

• Add support for Swift based on the following references:
  • Swift brush from syntaxhighlighter repo 6 years ago
  • A bunch of keywords/attributes were added to Swift in the past few years, and I added the commonly used ones with reference from the official Swift documentation
  • Swift implementation in two popular JS libraries:
    • Prism.js
    • Highlight.js

Please note that there are still some edge cases that the script doesn't cover, like multiline strings with extended string delimiters. I think we can support the most common Swift code usage then add/update more upon feature requests.

Testing instructions

1. Add or edit a post in Gutenberg editor
2. Add SyntaxHighlighter Code block
3. Select Swift as a language
4. Copy and paste some code from this gist or any other Swift examples
5. Save and preview
6. Make sure its syntax highlighted

Screenshot / Video

You can view some sample posts on my test site.

Gutenberg editor	post

[renatho]

Thank you for your PR, @jaclync!
Looks good and works well for me!