feat: implement static globals, explicit type casting, and conditional compilation

[LunaStev]

This PR introduces critical language primitives required for professional systems programming in Wave. Key additions include the as casting operator, global static variables, and a tiered platform abstraction layer using #[target] conditional compilation. These changes are supported by significant backend optimizations and a refactored standard library for better cross-platform portability.

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Key Changes

1. Core Language Features

• Explicit Type Casting (as): Implemented the as operator for explicit type conversions (e.g., value as i64). This is supported both in runtime expressions and during compile-time constant evaluation.
• Static Globals: Introduced the static keyword to declare global variables with persistent storage. These are correctly initialized in the LLVM data segment.
• Conditional Compilation (#[target]): Implemented a preprocessor-style attribute system. Using #[target(os="linux")] or #[target(os="macos")], developers can now write platform-specific code blocks within the same codebase.

2. LLVM Backend Optimizations

• Entry Block Allocas: Refactored the variable allocation strategy to use build_entry_alloca. By moving all local variable allocations to the function's entry block, we ensure better compatibility with LLVM's mem2reg pass, leading to more optimized machine code.
• Enhanced Constant Evaluation: Added support for integer sign-extension and pointer casts during compile-time evaluation.
• Cleanup: Removed legacy and unused image-generation logic (compile_ir_to_img_code) to streamline the backend.

3. Standard Library Portability

• Platform-Agnostic sys Imports: Refactored std::sys so users can import modules via generic paths like std::sys::fs rather than platform-specific paths like std::sys::linux::fs.
• C Compatibility: Updated errno handling to utilize __errno_location() for more reliable integration with standard C libraries.

4. CLI & Tooling

• Refined Build Flags:
  • Added -c for compile-only mode (producing object files).
  • Refined -o for specifying custom output paths.
• Tiered Platform Policy: Updated README.md to document the new support tiers for different operating systems and architectures.

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Example Usage

Static Globals, Casting, and Conditional Compilation:

static TOTAL_COUNT: i32 = 0;

#[target(os="linux")]
fun get_platform_name() -> str {
    var linux: str = "Linux";
    return linux;
}

#[target(os="macos")]
fun get_platform_name() -> str {
    var macos: str = "macOS";
    return macos;
}

fun main() {
    var x: i64 = 1000;
    var y: i64 = x as i32;

    println("Running on: {}", get_platform_name());
}

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Benefits

• Portability: The #[target] attribute and agnostic std::sys imports make it significantly easier to maintain cross-platform applications.
• Performance: Moving allocas to the entry block allows LLVM to perform better register allocation and dead-code elimination.
• Control: static variables and explicit casting provide the low-level memory and type control necessary for systems-level development.