feat: allow string return in keymap

[seblyng]

Problem:

In #2139 I outlined a bit about what issue I had, where expr mappings are being picked up before vim.on_key as opposed to regular mappings.

Solution:

Make it possible to return a string value to potentially opt out of the key being marked as typed.

With this I can for example do something like this:

["<CR>"] = {
    function(cmp)
        if not cmp.is_visible or require("blink.cmp.completion.list").get_selected_item() == nil then
            return
        end
        vim.schedule(function()
            cmp.accept()
        end)
        return vim.api.nvim_replace_termcodes("<ESC>a", true, false, true)
    end,
    "fallback",
}

This will make the callback invim.on_key not be invoked on <CR>