Support word 'SEARCH' from STRING

[davazp]

Implement SEARCH

Reference: https://www.taygeta.com/forth/dpans17.htm#17.6.1.2191

Before closing this ticket, please ensure:

• [ ] Is the run-time behaviour of the word implemented?
• [ ] Is the compile-time behaviour of the word implemented?
• [ ] Is the interpretation/top-level behaviour of the word implemented?
• [ ] Is the word covered by some tests?

This issue is part of an effort to make gbforth as close to the standard as possible. It was generated from the following index:

https://www.taygeta.com/forth/dpansf.html