fix: the config should load from source/_data too.

[snowyu]

No description provided.

[jiangtj]

This feature needs to be discussed @theme-next/next

First, NexT already supports reading the configuration under next.yml, for example:

├── source
│   └── _data
│       ├── next.yml

next.yml

foo:
  enable: true
  ...
bar:
  enable: false

And this PR will additionally get the configuration from other yml file (named by key)

├── source
│   └── _data
│       ├── foo.yml
│       └── bar.yml

foo.yml

enable: true
...

When there are many configurations in one key, splitting them into different files may be a good choice, but almost all in NexT are not long.

So in most cases, it is not used, and it will even make a lot of files under _data.

[snowyu]

This would be more nice for plugin development and avoid to put the plugin config into the main config file.