Update sass_api to 15.4.0 (#277)

Migrate nested color functions (#269)

Fixes #268.

Support --pkg-importer=node (#267)

* Support --pkg-importer=node

* Code review

Don't migrate plain CSS min and max (#263)

Fixes #262.

Add --built-in-only flag to module migrator (#259)

Also fixes some built-in functions that weren't being migrated.

Add legacy color function migrator (#260)

Fix null pointer errors in references (#256)

Fix operators regex to remove unnecessary parenthesis in SCSS variabl…

…es. (#254)

Keep interpolation in var CSS funcion (#250)

Use latest Dart Sass and release migrator 2.0.0 (#251)

- Deleted the obsolete `media-logic` migrator (breaking change bumps
  version to 2.0.0).
- Moved off of the tuple package to native Dart tuples.
- Fixed the `calc-interpolation` and `division` migrators to eliminate
  their use of the removed `CalculationExpression` AST node.
- Created a new `ScopedAstVisitor` that uses `Scope` to track Sass
  member declarations. `MigrationVisitor` and `_ReferenceVisitor`
  now extend this.
- Refactored the `division` migrator to use patterns.