[Feature Request] [stdlib] Make `String`'s `split` method accept `None` as the separator

[siitron]

Review Mojo's priorities

• [X] I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

Currently, String's split method does not support None as the separator to split on. In Python it's fully possible to perform splits such as some_string.split(None) and some_string.split(None, maxsplit=1) and some_string.split(None, 2). This is the same as not providing any sep at all.

What is your motivation for this change?

Python interoperability

Any other details?

No response