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

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

src/nvim/api/buffer.c

@@ -1273,6 +1273,63 @@ Object nvim_buf_call(Buffer buffer, LuaRef fun, Error *err)
   return res;
 }
 
+/// Create a new user command |user-commands| in the given buffer.
+///
+/// @param  buffer  Buffer handle, or 0 for current buffer.
+/// @param[out] err Error details, if any.
+/// @see nvim_add_user_command
+void nvim_buf_add_user_command(Buffer buffer, String name, Object command,
+                               Dict(user_command) *opts, Error *err)
+  FUNC_API_SINCE(9)
+{
+  buf_T *target_buf = find_buffer_by_handle(buffer, err);
+  if (ERROR_SET(err)) {
+    return;
+  }
+
+  buf_T *save_curbuf = curbuf;
+  curbuf = target_buf;
+  add_user_command(name, command, opts, UC_BUFFER, err);
+  curbuf = save_curbuf;
+}
+
+/// Delete a buffer-local user-defined command.
+///
+/// Only commands created with |:command-buffer| or
+/// |nvim_buf_add_user_command()| can be deleted with this function.
+///
+/// @param  buffer  Buffer handle, or 0 for current buffer.
+/// @param  name    Name of the command to delete.
+/// @param[out] err Error details, if any.
+void nvim_buf_del_user_command(Buffer buffer, String name, Error *err)
+  FUNC_API_SINCE(9)
+{
+  garray_T *gap;
+  if (buffer == -1) {
+    gap = &ucmds;
+  } else {
+    buf_T *buf = find_buffer_by_handle(buffer, err);
+    gap = &buf->b_ucmds;
+  }
+
+  for (int i = 0; i < gap->ga_len; i++) {
+    ucmd_T *cmd = USER_CMD_GA(gap, i);
+    if (!STRCMP(name.data, cmd->uc_name)) {
+      free_ucmd(cmd);
+
+      gap->ga_len -= 1;
+
+      if (i < gap->ga_len) {
+        memmove(cmd, cmd + 1, (size_t)(gap->ga_len - i) * sizeof(ucmd_T));
+      }
+
+      return;
+    }
+  }
+
+  api_set_error(err, kErrorTypeException, "No such user-defined command: %s", name.data);
+}
+
 Dictionary nvim__buf_stats(Buffer buffer, Error *err)
 {
   Dictionary rv = ARRAY_DICT_INIT;