Allow access to LSP completion item in sort comparator implementation

[tom-anders]

Feature Description

clangd has an extension where it adds it's own score to the LSP CompletionItem (based on things like number of usages, see https://clangd.llvm.org/extensions) - In nvim cmp the comparator had access to this field, so we could support the clangd extension like this: hrsh7th/nvim-cmp#277 (comment) (via entry.completion_item)

With blink, I don't see any way to access the CompletionItem - Any chance this could be added?