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.

test/functional/lua/ui_spec.lua

@@ -0,0 +1,46 @@
+local helpers = require('test.functional.helpers')(after_each)
+local eq = helpers.eq
+local exec_lua = helpers.exec_lua
+local clear = helpers.clear
+
+describe('vim.ui', function()
+  before_each(function()
+    clear()
+  end)
+
+
+  describe('select', function()
+    it('can select an item', function()
+      local result = exec_lua[[
+        local items = {
+          { name = 'Item 1' },
+          { name = 'Item 2' },
+        }
+        local opts = {
+          format_entry = function(entry)
+            return entry.name
+          end
+        }
+        local selected
+        local cb = function(item)
+          selected = item
+        end
+        -- inputlist would require input and block the test;
+        local choices
+        vim.fn.inputlist = function(x)
+          choices = x
+          return 1
+        end
+        vim.ui.select(items, opts, cb)
+        vim.wait(100, function() return selected ~= nil end)
+        return {selected, choices}
+      ]]
+      eq({ name = 'Item 1' }, result[1])
+      eq({
+        'Select one of:',
+        '1: Item 1',
+        '2: Item 2',
+      }, result[2])
+    end)
+  end)
+end)