fix: take into account prompt size in nui and builtin selects

Context

Width calculation logic in nui and builtin selects ignores prompt size. This makes it very easy to lose crucial information required to make a decision.

For example, here's what I see when running LspStart with the vtsls language server with master of stevearc/dressing.nvim:

And here's what I see with this PR:

Description

Initialize max_width (builtin) or line_width (nui) to the prompt length if it exists. Otherwise fallback to previous value of 1.

Test Plan

Used the manual select test with the following configuration:

-- Run this test with :source %

local function run_test(backend)
  local config = require("dressing.config")
  local prev_backend = config.select.backend
  config.select.backend = backend
  vim.ui.select({
    "first",
    "second",
    "third",
  }, {
    prompt = "Make a very informed selection after having collected all data and considered all viewpoints and opinions: ",
    kind = "test",
  }, function(item, lnum)
    if item and lnum then
      vim.notify(string.format("selected '%s' (idx %d)", item, lnum), vim.log.levels.INFO)
    else
      vim.notify("Selection canceled", vim.log.levels.INFO)
    end
    config.select.backend = prev_backend
  end)
end

-- Replace this with the desired backend to test
run_test("builtin")
--run_test("nui")

stevearc / dressing.nvim