Add support for Slang template

[hugopl]

Summary

Slang is basically a Slim template language for Crystal language, so the very same Slim parser works fine.

Slim template: https://github.com/slim-template/slim
Slang template: https://github.com/jeromegn/slang

Test plan

Create a simple slang file with some tailwind-css and check if the CSS is being extracted:

doctype html
html
  head
    title This is a title
  body.min-h-screen
    header.stick.top-0.z-10
      section.max-w-4xl.mx-auto.p-4.flex.items-center.justify-between
        h1.text-3xl.font-medium This is a slang file

To test it, get any slim template, rename the extension to .slang

Fixes #17851

[thecrypticace]

thanks!