feat(suggestions): export clear_preview and update_preview

[samiulsami]

These utility functions allow us to massively improve the UX for manually triggered suggestion workflows by loading suggestions in the background, and only showing them when triggered; greatly reducing the wait time required to generate a suggestion from scratch.

Example code snippet using the exported functions (with auto-trigger enabled):

		local copilot_suggestion = require("copilot.suggestion")

		vim.b.copilot_suggestion_hidden = true
		vim.api.nvim_create_autocmd({ "CursorMovedI", "InsertEnter", "InsertLeave" }, {
			group = vim.api.nvim_create_augroup("CopilotSuggestionHideGroup", { clear = true }),
			callback = function()
				if vim.b.copilot_suggestion_hidden then
					copilot_suggestion.clear_preview()
				end
				vim.b.copilot_suggestion_hidden = true
			end,
		})

		vim.keymap.set("i", "<C-o>", function()
			vim.b.copilot_suggestion_hidden = false
			if copilot_suggestion.is_visible() then
				copilot_suggestion.accept_line()
				return
			end
			copilot_suggestion.update_preview()
		end, { desc = "Accept Copilot suggestion (Line)" })

[AntoineGS]

You can ignore that last test failure, it does not seem related.
I'll make some tweaks as I am looking to move API endpoints to their own file to help future maintenance.
Thanks for the PR :)