Document str_split trailing separator behavior

[codeforester]

Summary

• document that str_split preserves a trailing empty field when input ends with the separator
• add BATS coverage for the trailing-separator case
• record the documentation/test pin in the changelog

Fixes #128.

Validation

• bats lib/bash/str/tests/lib_str.bats
• ./tests/validate.sh
• git diff --check