ex_cmds: port :eval

Cherry-picked from patch v8.1.1807.
Required for patch v8.2.2761.

src/nvim/ex_cmds.lua

@@ -927,6 +927,12 @@ module.cmds = {
     addr_type='ADDR_NONE',
     func='ex_edit',
   },
+  {
+    command='eval',
+    flags=bit.bor(EXTRA, NOTRLCOM, SBOXOK, CMDWIN),
+    addr_type='ADDR_NONE',
+    func='ex_eval',
+  },
   {
     command='ex',
     flags=bit.bor(BANG, FILE1, CMDARG, ARGOPT, TRLBAR),