config.register_option allows registering invalid options
```python
>>> import pandas as pd
>>>
>>> pd.__version__
'0.26.0.dev0+684.g953757a3e'
>>>
>>> from pandas._config import config
>>>
>>> config.register_option("a.b.c", 1)
>>>
>>> config.get_option("a.b.c")
1
>>>
>>> config.register_option("a.b.c", 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\simon\pandas\pandas\_config\config.py", line 445, in register_option
    raise OptionError(msg.format(key=key))
pandas._config.config.OptionError: "Option 'a.b.c' has already been registered"
>>>
>>> config.register_option("a.b", {"c": 2})
>>>
>>> config.get_option("a.b.c")
2
>>>
```