Implement the `sassify` filter

[osteele]

Jekyll provides both sassify and scssify Liquid filters to convert Sass/SCSS code into CSS:

• scssify: Converts SCSS syntax to CSS (already implemented in gojekyll)
• sassify: Converts Sass indented syntax to CSS (not yet implemented)

Example usage:

{{ "div { color: $primary-color; }" | sassify }}

Gojekyll has implemented scssify using the Dart Sass transpiler (see filters/filters.go line 125), but sassify currently returns a warning (line 124) and is not functional.

Implementation notes:

• Should use the same Dart Sass transpiler as scssify
• Need to pass the appropriate syntax flag to distinguish Sass vs SCSS
• Listed as missing in README.md lines 210-211