641: Allow whitespace on either side of main header delimiter

[lindsay-stevens]

Closes #641

Why is this the best possible solution? Were any other approaches considered?

A user might put white space on either side of the double colon delimiter, so it's stripped off the tokens after splitting. Trailing or leading white space isn't needed either so it's stripped off both ends of the tokens.

What are the regression risks?

Low risk, it should just fix the problem in #641. I updated the code branch for single colons, because although that usage has been deprecated since what seems like 2012, it's different question to #641 (e.g. should code for the single colon case be removed? raise an error instead? or a warning? etc).

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run python -m unittest and verified all tests pass
• run ruff format pyxform tests and ruff check pyxform tests to lint code
• verified that any code or assets from external sources are properly credited in comments