Support word '[THEN]' (bracket-then) from TOOLS EXT

[davazp]

Implement [THEN]

Reference:
https://www.taygeta.com/forth/dpans15.htm#15.6.2.2533

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