Fix wb single byte write behavior

[jukuisma]

• Mark this if you consider it ready to merge
• I've added tests (optional)
• I wrote some lines in the book (optional)

Description

Fix a bug in single byte wb commands and add some new test cases that catch the old bug. Related to: #23405, but doesn't implement the multibyte wb commands yet.

[jukuisma]

TODO:

• Update (neo)?vim configs to use tabs instead of spaces in C
• Amend commit messages and write PR description
• Use bitwise or instead of +=, which should also work now that the shifts have been fixed
• Consider dropping the extra shift variable to make the function even more readable

[trufae]

Good catch and fix, TDD all the way!