Reject multi-character split separators

[Rokesh2008]

Fixes #14649.

The custom strings.split.split implementation compares one input character at a time, so a multi-character separator is silently ignored and returns incorrect output. This PR:

• raises ValueError("separator must be a single character") for invalid separators
• preserves existing single-character separator behaviour
• adds a doctest and pytest regression coverage

Tests:

• python -m pytest tests/test_split.py -q (2 passed)
• python -m doctest strings/split.py
• git diff --check

[cclauss]

The doctests are good enough. We do not need any tests added here.