v0.2.1

v0.2.1: Built-in variables, return values, and math operations

Features:
- Built-in variables: __FILE__, __LINE__, __OS__, __USER__, __TEMP__, __MACHINE__, __DOMAIN__, __PROCESSOR_COUNT__, __ARGS__
- Return statement in functions with value propagation
- Math operations: math.add, math.subtract, math.multiply, math.divide, math.round, math.floor, math.ceil
- String operations: string.concat, string.format
- Syntax: env set var=funcname(args) for capturing return values

Tests:
- test-builtin-vars.wflow - demonstrates all built-in variables
- test-return-values.wflow - return values, math, and complex functions