Beta: Migratation to Sass Modules

This is a major update, moving over to Sass modules,
and removing some features that no longer seem necessary.

• BREAKING: Requires Sass modules, and other recent features of Dart Sass.
  Legacy versions of Sass (Node & Ruby) are no longer supported

• BREAKING: The old "init" module, which provided light-weight
  browser normalization, is no longer included. We now recommend
  using CSS Remedy.

• BREAKING: The old "core" module has been broken into individual modules
  for "utilities" (extending several Sass modules),
  "tokens" (the map syntax parser),
  "variables" (converting Sass variables & maps to CSS custom properties),
  and "ratios" (for typographic scales and layout aspect ratios)

• Sass Utilities:

  • NEW: These functions are now available directly,
    as well as being registered in the map syntax

• Variables:

  • NEW: ident() function adds -- and an optional prefix to any string,
    in order to generate a custom property identifier.
  • NEW: custom-props() mixin generates custom properties
    for every key/value pair in a map.

• Tokens:

  • NEW: A module just for the custom map syntax parser & function registration
  • BREAKING: The $functions map no longer accepts alias references
    or any other aspects of the token syntax
  • BREAKING: Ratios are no longer first-class adjustments
    (like functions) in the token syntax
  • BREAKING: tokens.$handle-missing-keys defaults to null,
    and no longer supports the legacy silent option.
  • BREAKING: get-token() is renamed get(),
    and only accepts bare token names (no # prefix),
    including the outer->inner nested token syntax.
  • NEW: The new compile() function provides direct access to
    parse & resolve an arbitrary token value
    (including aliases with # prefix),
    rather than calling a specific token by name.
  • NEW: All the built-in Sass module functions are registered by default,
    as <module-name>.<function-name>
  • NEW: has-token() function checks a map
    to see if the given token is defined,
    and supports the outer->inner nested token syntax.

• Ratios:

  • NEW: Built-in named ratios are now available as individual variables,
    (without the _ prefix that is applied in token maps)
  • NEW: is-ratio() function can be used to type-check ratios,
    including ratio tokens.

• Scale:

  • NEW: scale() function applies a modular scale to any value,
    allowing you to move up or down the scale any number of steps.

• Layout:

  • BREAKING: Removed global-box-sizing
  • BREAKING: Removed clearfix
  • BREAKING: Removed fluid-ratio
  • BREAKING: Removed position. Can achieve the majority of functionality in CSS using
    Inset