Function hover_at 

pub fn hover_at(
    ast: &Ast,
    interner: &ThreadedRodeo,
    file_id: FileId,
    byte: u32,
) -> Option<HoverContent>

Find an item under the cursor and produce hover content for it.

file_id constrains the search to one file: items from other files (via the merged AST) are skipped because their spans live in another FileId namespace.