feat: use vim.uv.spawn instead of vim.fn.jobstart

[delphinus]

The current code uses vim.schedule_wrap to call vim.fn.jobstart for running rg. This can be replaced by uv.spawn that can be called “in fast event”. This makes it run rg as soon as cmp starts to complete.

But this change has 2 problems.

• vim.fn.json_decode cannot be called in fast event. We should always use vim.json.decode, so we must remove support for Neovim older than v0.6.
• uv.spawn always run a process directly (without shell). Now additional_arguments option should be an array instead of string.

I think benefits we can get by this change are larger than these breaking changes.

[lukas-reineke]

Does this actually make a different to the performance?
Can you profile this?