fix(lua): no omni/cmdline completion for vim.env (#33044)

Problem:
- `:lua vim.env.<Tab>` does not show completion of environment variables
- Meanwhile, `:let $<Tab>` does show completion of environment variables

Solution:
- Fix it

runtime/lua/vim/_editor.lua

@@ -1031,7 +1031,7 @@ function vim._expand_pat(pat, env)
   end
 
   -- Completion for dict accessors (special vim variables and vim.fn)
-  if mt and vim.tbl_contains({ vim.g, vim.t, vim.w, vim.b, vim.v, vim.fn }, final_env) then
+  if mt and vim.tbl_contains({ vim.g, vim.t, vim.w, vim.b, vim.v, vim.env, vim.fn }, final_env) then
     local prefix, type = unpack(
       vim.fn == final_env and { '', 'function' }
         or vim.g == final_env and { 'g:', 'var' }
@@ -1039,6 +1039,7 @@ function vim._expand_pat(pat, env)
         or vim.w == final_env and { 'w:', 'var' }
         or vim.b == final_env and { 'b:', 'var' }
         or vim.v == final_env and { 'v:', 'var' }
+        or vim.env == final_env and { '', 'environment' }
         or { nil, nil }
     )
     assert(prefix and type, "Can't resolve final_env")

test/functional/lua/command_line_completion_spec.lua

@@ -269,6 +269,18 @@ describe('nlua_expand_pat', function()
       }
       eq(expected, actual)
     end)
+
+    it('vim.env', function()
+      exec_lua [[
+        vim.env.NLUA_ENV_VAR = 'foo'
+      ]]
+      local actual = get_completions('vim.env.NLUA')
+      local expected = {
+        { 'NLUA_ENV_VAR' },
+        #'vim.env.',
+      }
+      eq(expected, actual)
+    end)
   end)
 
   describe('completes', function()