Add snapshot tests for SassProcessor

Tests cover CSS output (preserveIcssExports true/false), .d.ts generation,
Sass-specific features (variables + nesting, @mixin, @extend + placeholders),
JS shim generation (commonjs/esnext, module/global), multiple output folders,
postProcessCssAsync callback, exportAsDefault: false, and error reporting.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: iclanton