interpreter: implement missing builtin functions (18 of 23 missing)

[Prestgg1]

Goal

Implement all 23 builtin functions in the interpreter runtime.

Current State

interpreter/src/runner/builtin/mod.rs only implements 5 of 23:

Implemented

• Print, Input, Len, LastWord (broken - calls exit(1)), Sum

Missing Builtins

• Number - convert to number
• Range - generate range list
• Trim - trim string whitespace
• Sqrt - square root
• Timer - timing function
• Max - maximum of two values
• Mod - modulo operation (may overlap with % operator)
• Min - minimum of two values
• Round - round float
• Floor - floor float
• Ceil - ceiling float
• Zig - run Zig code inline
• StrUpper - uppercase string
• StrReverse - reverse string
• StrLower - lowercase string
• Allocator - allocator function
• ConvertString - string conversion

All of these fall to _ => Value::Void which silently does nothing.