fix(split): support multi-character separators

[MD-Mushfiqur123]

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The custom split function in strings/split.py compares each character in the input string against the full separator string, which means multi-character separators such as "--" are never matched and the string is returned unsplit.

Fix

Replace character-by-character iteration with substring matching at each position, comparing string[index:index + separator_length] against the separator. This correctly handles separators of any length.

Testing

• All existing doctests pass
• Added doctests for multi-character separators: "--" and "##"

Fixes #14649