Implement PrependIfMissing method.

**Is your feature request related to a problem? Please describe.**
Prepends a prefix to the start of the String if not present.
This method porting from Apache commons lang.

Inspect the repository's existing String class extensions and any available test or build entry points; no specific file or test is named in the issue. Implement the requested prefix behavior consistently with neighboring methods, then verify the project builds and the method handles an already-present prefix.

Written by the indexing model from the issue text.