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.

src/nvim/lua/vim.lua

@@ -108,6 +108,9 @@ setmetatable(vim, {
     elseif key == 'diagnostic' then
       t.diagnostic = require('vim.diagnostic')
       return t.diagnostic
+    elseif key == 'ui' then
+      t.ui = require('vim.ui')
+      return t.ui
     end
   end
 })