Set up a data source for Sass for hover info, completions

[wkillerud]

From what I can tell there's no machine-readable format for the documentation. Potential sources for inspiration:

• https://github.com/sass/sass-site/blob/main/tool/generate-module-metadata.ts
• https://github.com/wkillerud/some-sass/blob/main/packages/language-services/src/facts/sass.ts#L45

[wkillerud]

Another thing perhaps worth exploring here is the concept of a virtual file for the built-in Sass modules (wkillerud/some-sass#256 (comment)). It might open for a nice experience with navigation, inline documentation, and could potentially simplify how we implement other features (no need for special handling of built-ins, maybe).