feat(api): implement nvim_{add,del}_user_command

Add support for adding and removing custom user commands with the Nvim
API.

src/nvim/api/keysets.lua

@@ -33,6 +33,18 @@ return {
   get_commands = {
     "builtin";
   };
+  user_command = {
+    "addr";
+    "bang";
+    "bar";
+    "complete";
+    "count";
+    "desc";
+    "force";
+    "nargs";
+    "range";
+    "register";
+  };
   float_config = {
     "row";
     "col";