fix(gx): visual selection, expand env vars

---
Rejected experiment: move vim.ui.open() to vim.env.open()

Problem:
`vim.ui` is where user-interface "providers" live, which can be
overridden. It would also be useful to have a "providers" namespace for
platform-specific features such as "open", clipboard, python, and the other
providers listed in `:help providers`. We could overload `vim.ui` to
serve that purpose as the single "providers" namespace, but
`vim.ui.nodejs()` for example seems awkward.

Solution:
`vim.env` currently has too narrow of a purpose. Overload it to also be
a namespace for `vim.env.open`.

diff --git a/runtime/lua/vim/_meta.lua b/runtime/lua/vim/_meta.lua
index 913f1fe20348..17d05ff37595 100644
--- a/runtime/lua/vim/_meta.lua
+++ b/runtime/lua/vim/_meta.lua
@@ -37,8 +37,28 @@ local options_info = setmetatable({}, {
   end,
 })

-vim.env = setmetatable({}, {
-  __index = function(_, k)
+vim.env = setmetatable({
+  open = setmetatable({}, {
+      __call = function(_, uri)
+        print('xxxxx'..uri)
+        return true
+      end,
+      __tostring = function()
+        local v = vim.fn.getenv('open')
+        if v == vim.NIL then
+          return nil
+        end
+        return v
+      end,
+    })
+  },
+  {
+  __index = function(t, k, ...)
+    if k == 'open' then
+      error()
+      -- vim.print({...})
+      -- return rawget(t, k)
+    end
     local v = vim.fn.getenv(k)
     if v == vim.NIL then
       return nil

test/functional/lua/ui_spec.lua

@@ -1,5 +1,6 @@
 local helpers = require('test.functional.helpers')(after_each)
 local eq = helpers.eq
+local matches = helpers.matches
 local exec_lua = helpers.exec_lua
 local clear = helpers.clear
 local feed = helpers.feed
@@ -11,8 +12,7 @@ describe('vim.ui', function()
     clear()
   end)
 
-
-  describe('select', function()
+  describe('select()', function()
     it('can select an item', function()
       local result = exec_lua[[
         local items = {
@@ -47,7 +47,7 @@ describe('vim.ui', function()
     end)
   end)
 
-  describe('input', function()
+  describe('input()', function()
     it('can input text', function()
       local result = exec_lua[[
         local opts = {
@@ -130,4 +130,18 @@ describe('vim.ui', function()
     end)
 
   end)
+
+  describe('open()', function()
+    it('validation', function()
+      exec_lua[[vim.ui.open('non-existent-file')]]
+      matches('vim.ui.open: command failed %(%d%): { "[^"]+", "non%-existent%-file" }', eval('v:errmsg'))
+
+      exec_lua[[
+        vim.fn.has = function() return 0 end
+        vim.fn.executable = function() return 0 end
+      ]]
+      exec_lua[[vim.ui.open('foo')]]
+      eq('vim.ui.open: no handler found (tried: wslview, xdg-open)', eval('v:errmsg'))
+    end)
+  end)
 end)