feat(ui): add vim.ui.select and use in code actions (#15771)

Continuation of https://github.com/neovim/neovim/pull/15202

A plugin like telescope could override it with a fancy implementation
and then users would get the telescope-ui within each plugin that
utilizes the vim.ui.select function.

There are some plugins which override the `textDocument/codeAction`
handler solely to provide a different UI. With custom client commands and
soon codeAction resolve support, it becomes more difficult to implement
the handler right - so having a dedicated way to override the picking
function will be useful.

runtime/lua/vim/ui.lua

@@ -0,0 +1,36 @@
+local M = {}
+
+--- Prompts the user to pick a single item from a collection of entries
+---
+---@param items table Arbitrary items
+---@param opts table Additional options
+---     - prompt (string|nil)
+---               Text of the prompt. Defaults to `Select one of:`
+---     - format_item (function item -> text)
+---               Function to format an
+---               individual item from `items`. Defaults to `tostring`.
+---@param on_choice function ((item|nil, idx|nil) -> ())
+---               Called once the user made a choice.
+---               `idx` is the 1-based index of `item` within `item`.
+---               `nil` if the user aborted the dialog.
+function M.select(items, opts, on_choice)
+  vim.validate {
+    items = { items, 'table', false },
+    on_choice = { on_choice, 'function', false },
+  }
+  opts = opts or {}
+  local choices = {opts.prompt or 'Select one of:'}
+  local format_entry = opts.format_entry or tostring
+  for i, item in pairs(items) do
+    table.insert(choices, string.format('%d: %s', i, format_entry(item)))
+  end
+  local choice = vim.fn.inputlist(choices)
+  if choice < 1 or choice > #items then
+    on_choice(nil, nil)
+  else
+    on_choice(items[choice], choice)
+  end
+end
+
+
+return M