Support for String-based API to Parse SCSS

[mpiniarski]

I propose adding support for a string-based API.
This would enable integration with custom file formats and build processes, allowing SCSS to be passed directly as a string. It would be particularly useful in cases where styles are embedded in different file formats.

Example Use Case:

• Extract SCSS from .vue files (single file components) and pass it to find-unused-sass-variables.