Move to Sass Modules (🚨Requires Dart Sass🚨)

• 🚨 BREAKING: Switch to Dart Sass with Sass module system, dropping support for Node Sass.
• 🚨 BREAKING: Drop support for node < 10
• 🚨 BREAKING: Rename $true-terminal-output setting to $terminal-output when importing as a module (with @use). Projects not using Sass modules can still @import '<path>/sass-true/sass/true' and access the setting as $true-terminal-output
• 🎉 FEATURE: Added _index.scss at the project root, for simpler import path: @use '<path>/sass-true'
• 🎉 FEATURE: New sass/_throw.scss module provides:
  • error() function & mixin for establishing "catchable" errors
  • global $catch-errors toggles how error() output is handled
• 🎉 FEATURE: Support testing content properties which include a curly brace.
• 🛠 Update dependencies