feat: add STRINGIFY() builtin macro

[aspizu]

Summary

Adds a new builtin function-like macro STRINGIFY() to the preprocessor.

Behaviour

STRINGIFY(expr) expands to a string literal whose contents are the space-joined token text of the argument expression, before any further macro expansion (similar to C's # stringification operator).

Example

%define MY_VAR 42
say STRINGIFY(MY_VAR + 1);   // produces the string "MY_VAR + 1"

Implementation

• Added substitute_stringify method in src/pre_processor.rs, modelled after the existing substitute_concat.
• Hooked it into the process loop alongside substitute_concat.

[faretek1]

What is this actually meant to be used for?

[aspizu]

This is primarily intended for use in testing macros. For example, a TEST(expr) macro could use STRINGIFY to include the original expression text in its output message, so TEST(A+B) could print smth like "Assertion failed: A+B".