Add REXX language support (gen_scripts)

[darkn3rd]

Add REXX, via the Regina REXX interpreter (https://regina-rexx.sourceforge.io/), as a new language in lessons/gen_scripts/, alongside the existing awk/groovy/perl/php/python/ruby/tcl.

Based on tcl/ as the reference (interpreted, no build/link step - just scripts/ + Rakefile + psakefile.ps1, no Makefile):

• New lessons/gen_scripts/rexx/ directory: Rakefile (imports shared testbox.rake), psakefile.ps1, scripts/ with one .rexx/.rex file per lesson (~60 files mirroring tcl/'s/perl/'s set), README.md
• Package availability confirmed: Ubuntu (apt: regina-rexx), Cygwin (cyg: regina-rexx), macOS (brew: regina-rexx)
• Windows: no Chocolatey package found - likely needs a direct-download install script (from sourceforge) rather than a choco: entry, similar to how other non-packaged tools in this project are handled
• MSYS2: no mingw-w64-* regina-rexx/rexx package found in a quick check - needs confirming; if genuinely unavailable, may need to point at the Windows install via a PATH script, matching the existing msys2_groovy_path/cygwin_groovy_path pattern
• Add REXX to lessons/gen_scripts/{Brewfile,choco.config,cygwin_pkgs.txt,msys2_pkgs.txt,ubuntu2204_pkgs.txt} (Windows/MSYS2 pending the above)
• Add a REXX entry to the General Scripts area in scriptbox/scripts/verify_commands.rb's AREAS
• lessons/gen_scripts/README.md update

This is a full new-language port (~60 lesson files), so likely worth breaking into smaller PRs rather than one large one.