Add REXX interpreter (copying RexxJS's grammar/features)

[paul-hammant]

Implements a comprehensive REXX interpreter for ChrysaLisp with strong RexxJS grammar compatibility, featuring ARexx-style IPC via ADDRESS/PORTS and modern REXX language extensions.

Core Language Features:

• Variable assignment (classic and LET keyword)
• String interpolation with {variable} syntax
• || string concatenation operator
• HEREDOC multi-line strings (<<DELIMITER ... DELIMITER)
• Escape sequences (\n, \t, \r, \, ")
• Comments (// C-style and -- REXX-style)

Control Flow:

• IF/THEN/ELSE (single-line and multi-line blocks)
• SELECT/WHEN/OTHERWISE multi-way branching
• DO loops (counted with TO/BY, WHILE, FOREVER, OVER)
• LEAVE and ITERATE loop control
• EXIT with return codes

Advanced Features:

• CALL/RETURN subroutines with local scopes
• SIGNAL for computed jumps
• INTERPRET for dynamic code execution
• PARSE (VAR, VALUE, ARG) for string parsing
• Stem variables (REXX arrays with .0 count convention)

String Functions (40+):

• UPPER, LOWER, LENGTH, SUBSTR, PROPER, TRIM
• PAD_START, PAD_END, CENTER, SPACE
• STARTS_WITH, ENDS_WITH, INCLUDES
• IS_ALPHA, IS_NUMERIC, IS_ALPHANUMERIC
• CHANGESTR, WORDPOS, DELWORD, SUBWORD, ABBREV
• SLUG, REPEAT, INDEXOF, VERIFY, COMPARE, COPIES
• C2X, X2C, X2D, D2X, X2B, B2X (conversion functions)

Math Functions:

• ABS, SIGN, MAX, MIN
• MATH_CEIL, MATH_FLOOR, MATH_ROUND, MATH_SQRT, MATH_POWER

Array Operations:

• ARRAY, PUSH, POP, JOIN
• DO OVER iteration (RexxJS extension)

JSON Support:

• JSON_STRINGIFY, JSON_PARSE
• CSV_TO_JSON, JSON_TO_CSV

IPC System (ARexx-style):

• ADDRESS statement for target switching
• PORT system for inter-process communication
• SEND/RECEIVE mailbox operations
• ADDRESS SYSTEM for ChrysaLisp native Lisp execution
• Live IPC demo with ECHO service

Files Created:

• apps/rexx/interpreter.inc (2000+ lines core interpreter)
• apps/rexx/ports.inc (IPC implementation)
• apps/rexx/app.lisp, actions.inc, widgets.inc (GUI)
• apps/rexx/tests/test_suite.rex (59 automated tests)
• apps/rexx/examples/ (20+ example programs)
• apps/rexx/REXXJS_COMPATIBILITY.md (compatibility analysis)
• apps/rexx/echo_service.lisp, system_port.lisp (IPC services)

RexxJS Compatibility:

• ~80% core language compatibility
• 100+ functions implemented
• Focus on ChrysaLisp-appropriate features
• Deferred: HTTP, REQUIRE, file I/O, DOM (environment-dependent)

Test Coverage:

• 59/59 automated tests passing
• Examples for all major features
• Comprehensive test suite with live IPC demonstrations

Documentation:

• README.md with feature overview
• REXXJS_COMPATIBILITY.md with detailed analysis
• Inline documentation throughout
• Example programs demonstrating all features

This implementation enables REXX scripting within ChrysaLisp with modern RexxJS syntax extensions and native IPC integration.

[paul-hammant]

This was about ARexx's ADDRESS/PORTS and IPC, when I started it